Suppose the Habsburgs don't inherit the Spanish throne - without Spanish involvement in the conflict, how would things have turned out in the greater scheme of things?
Netherlands would be the richest land under imperial control and likely the seat (or one of the seats) of either the Emperor or his heir...Well before the 30YW would be a major change in the Italian Wars: Aragonian claim for the Naples would have nothing to do with Burgundian inheritance and Hapsburg interest in Northern Italy. The same goes for the military effort: France would be able to deal with the Spaniards and Germans separately. Financial situation for the Hapsburgs would be worse without American gold and silver and Spain would be probably better off in that regard because it has to finance a much smaller military and political activities. Without Spanish ‘global’ policies French Wars of Religion may end faster making France stronger by the time of the 30YW.
Taking into an account that the Netherlands were Hapsburg possessions, Spain does not have any interest in the 30YW and there is no help with troops and money to the Hapsburgs on the initial stage of war which means that even with the leadership they had, the Czechs may force Hapsburgs to make a peace. If this is not happening, there are no Spanish troops on the Swedish stage of war. France is much better positioned to take all the areas on their northern border they got in OTL. BTW, are the Netherlands rebellious? Strictly German Hapsburgs would have a hard time to fight them for 80 years even if just because they did not have money.
Netherlands would be the richest land under imperial control and likely the seat (or one of the seats) of either the Emperor or his heir...
But Habsburg would still be allied and kinsmen of the Spanish rulers (and the union of the two kingdoms was only during Karl V’s kingdom and can very well be butterflied with either the survival of Juan, Prince of Asturias or Miguel da Paz or with Ferdinand of Austria as King of Castile and Aragon after his grandfather (aka Karl V never received the Spanish Crowns)
I think it will be difficult to exclude Spain from the Italian Wars and any religious wars given that before Ferdinand died they were already established in Naples. Spain my play a reduced role in the later Italian wars which may mean that France is more successful but any threat to the Papal States will likely bring in Spain on the Papal side.
No 80 years war in the Netherlands for Spain will also mean it is less stretched financially and better placed to intervene decisively in the French Wars of Religion.
In fact France may become the battleground for religion rather than Germany (or more correctly before Germany). Also Spain will be in a better position to prolong its annexation of Portugal.
Assuming a Catholic win in France of some description I would still expect a Spanish army to be involved in the alt-30 years war.
But this army would be closer to the Swedes in its raison d'etre - late to the war and primarily there as "mercenaries" for the cause of their religion.
A more Catholic leaning France may also decide to intervene on the Catholic side but all this will do is probably extend the war to the Netherlands (and possibly postpone the English Civil War??).
Netherlands would be the richest land under imperial control and likely the seat (or one of the seats) of either the Emperor or his heir...
But Habsburg would still be allied and kinsmen of the Spanish rulers (and the union of the two kingdoms was only during Karl V’s kingdom and can very well be butterflied with either the survival of Juan, Prince of Asturias or Miguel da Paz or with Ferdinand of Austria as King of Castile and Aragon after his grandfather (aka Karl V never received the Spanish Crowns)
The Netherlands had been the richest land in Europe but this did not prevent Maximillian, Charles V and Phillips II from being permanently out of money: there were traditional rules related to the taxation.
Not sure how exactly the Hapsburgs in that AH end up as close kinsmen of the Spanish rulers and which “two kingdoms” you are talking about (HRE was not a “kingdom”) but in OTL Spain had a vested interest in the 30 YW, security of the ‘Spanish Road’, which in this AH is absent. Being a relative does not automatically obliges a ruler to throw all resources of his kingdom into a major war in which he does not have interests. Not to mention that if Spain does not have the Netherlands (Belgium) there are no reinforcements on the Bohemian stage of war (they came from the Netherlands).
The Imperial interest in Milan will be always first give back the Duchy to the Storzas but if that line will be extinct like OTL you can be almost sure who the Emperor will name one of his sons as new Duke.Indeed it would not butterfly away the Habsburg-Trastamare matches and their joint concerns about French power. There were valid reasons these marriages came about. Italy is a concern, they both had. Aragon had traditional claims and holdings in the south of Italy; the Habsburg as Emperor of the HRE had claims (it were (albeit mostly nominal) Imperial fiefs on northern Italy, however with France already neighbouring their Burgundian Lands, France also threatening their Austrian Hereditary Lands from northern Italy is a very dangerous threat indeed. Like OTL the Habsburgs probably would want to end up with the duchy of Milan (if the are really victorious ITTL it would be a fitting compensation for the Burgundian Lands seized by France), more likely the would have to accept, that the best chance to get a strong enough alliance to remove France from Italy is that they back a Sforza restoration in Milan.
As for the Habsburg financial capacities, well there are the Low Countries with Antwerp (Antwerpen), Ghent (Gent), Bruges (Brugge), Amsterdam etc., but let's not forget the Fuggers and their Austrian Lands did have some profitable banks. Besides given their aligned interests, Spain might be willing to bankroll, well give loans to, their Habsburg kinsmen for Italian campaigns. The Low Countries, though notorious hard to tax, well when facing a French threat the Estates General will give funds for the protection of these lands.
It also gives some benefits for Habsburg politics in the Empire, their main focus is actually there and ITTL Charles V, or maybe his father Philip the Handsome will be Emperor first ITTL, will be seen as a much more native Emperor. In fact he might be able to handle to Reformation sooner and more effective, maybe or likely not stopping it entirely, but at least the Catholic church ITTL will be more supportive earlier on, if ironically the Emperor is weaker (OTL they felt Charles V was too strong and at first thought those protestant rebels could be useful to weaken him or at least distract him for a while).
@alexmilman: I don't think, that the Habsburgs would let the situation in the Netherlands deteriorate in such a way, if only because it is one of the 'jewels in their crown'. Many think that the Dutch Rebellion was purely religious, well they are wrong, Catholics and Protestants alike were unhappy about the political situation, and while OTL Charles V was born in Ghent and raised in Brussels/Mechelen (or is it Malines in English), his son Philip II was born in Spain and never understood the region. ITTL the Habsburgs can't afford to ignore these lands. Especially if the Habsburgs still end up with Bohemia and Hungary ITTL, then I can see the Emperor mostly residing in Vienna or Prague, but his heir, most often the King of the Romans, could end up residing at the Koudenberg Palace in Brussels.
A more native Emperor might get more support from Imperial vassals to protect Imperial lands, well at least those north of the Alps against foreign invaders.
Sure Charles V or Philip OTL and ITTL was mostly Francophone and better in Dutch than German, but the Luxembourg dynasty was also more Francophone than German, but it were native dynasties.
OTOH at the Imperial level, the Austrian Hereditary Lands combined with the Burgundian Inheritance and possibly the Lands of the Bohemian Crown, would give the Emperor a powerbase not seen since the days of the Hohenstaufen dynasty. Some bigger houses might want to weaken that, smaller houses might actually like a more effective Emperor against said bigger houses.
I was just saying how preventing Charles’ inheritance of Spain (and so the union of the realms) in first place as none of the situation I suggested would see Charles on the Spanish throne so I can not understand why Charles “would leave Spain to somebody who is not a Habsburg” as you are saying.
3) Ferdinand of Austria is the younger brother of Charles and an Habsburg but him as King of Spain (and is possible only instead and not after Charles) still satisfy the requisite of preventing the union between Spain and Austria because Ferdinand was born and raised in Spain ...
... so I can not see OTL level of cooperation between Spain and Austria-Burgundy.
@alexmilman: I don't think, that the Habsburgs would let the situation in the Netherlands deteriorate in such a way, if only because it is one of the 'jewels in their crown'. Many think that the Dutch Rebellion was purely religious, well they are wrong, Catholics and Protestants alike were unhappy about the political situation,
OTOH at the Imperial level, the Austrian Hereditary Lands combined with the Burgundian Inheritance and possibly the Lands of the Bohemian Crown, would give the Emperor a powerbase not seen since the days of the Hohenstaufen dynasty. Some bigger houses might want to weaken that, smaller houses might actually like a more effective Emperor against said bigger houses.
Don't forget England, which traditionally has been a large trading partner of the Low Countries. France too, though it also had a history of wanting to expand into this region. Financing a war to fight France in Italy is different from having to finance a war against France directly threatens the Low Countries. In case of the latter, support won't be an issue, in case of the former, well getting more than legally obligated will be a lot harder (but not impossible). ITTL the Habsburgs are probably facing a more sympathetic Papacy and thus Catholic side in the HRE will be able to respond sooner and formulate an answer. The Habsburgs had long advocated for a Council of Trent, before it happened IOTL. Conflicts will probably still arise, OTOH Dutch protestants generally are Calvinists, and not Lutherans like in Germany. If ITTL France still ends up backing the Protestants in Germany, then IMHO it's not a stretch to see Spain supporting the Catholic side, though not like IOTL.And financial situation as well. The Netherlands had been forced to financed the endless wars with France, the Ottomans and German Protestants. Not to mention that at least in the cases of France and Germany the wars were against the traditional trade partners. Plus, there was a matter of shifting power: while traditionally a lot of it was broken by the regions and in the hands of a local nobility and rich merchants, Charles V and especially Phillip II tried to concentrate power in the hands of a central government (which, strictly speaking, was historically "progressive") through their own appointees. "Under the governorship of Mary of Hungary (1531–1555), traditional power had for a large part been taken away both from the stadtholders of the provinces and from the high noblemen, who had been replaced by professional jurists in the Council of State." (https://en.wikipedia.org/wiki/Dutch_Revolt#Taxation) Of course, the locals were opposite to happy.
Well, adding the Netherlands to the OTL list would be a bonus but as far as a power base is involved, the Hapsburgs ruled each of these territories as a separate entity with a lot restrictions (at least in the cases of Bohemia and in this AH the Netherlands) on what they could do legally and what they could get away with. A big difference between OTL and this AH is that, unlike OTL Phillip II, his AH equivalent does not have a ready enforcement tool (Spanish troops), which was deployed in OTL (to turn problem into a full scale disaster). Not to mention that a balance between Catholics and Protestants would be quite different from one in OTL Spanish "empire". So, short of a complete insanity, alt-Phillip () would have to look for a compromise solution or risk a major revolt across a big part of his territories. And if he is exercising a flexible religious policy then one of the causes for the 30YW is gone (and another, maintenance of the Spanish Road and resulting Spanish involvement is not there by definition).
The Imperial interest in Milan will be always first give back the Duchy to the Storzas but if that line will be extinct like OTL you can be almost sure who the Emperor will name one of his sons as new Duke.
And yes, no way who the situation in Netherlands will go down like OTL as that rebellion was against the foreign Philip II and in a world where Burgundy is united only to Austria (plus maybe Bohemia and Hungary) they will be either one of the main seats of the Holy Roman Emperor or the seat of his heir (and will be ruled directly by them)