The constitutional arrangements could perhaps borrow from the HRE when it was still effective. That too was an Empire consisting of a number of semi independent states. Not all of the same race or language (back then) . And that model would of course be very familiar to men of the time.
I don't think the parts of the empire can fight each other - as I said, how can the King fight himself. Though the components of the HRE managed it!
Whether they would get along harmoniously, I think more dubious. ISTR that some of the American (British) colonies ended up practically at war with one another at some point ! The connections between the Low country and England are old, and strong, and they share a similar economic rationale. Those between either of the former and Spain, much weaker and more recent , and the economies very different (IMHO far too little attention is given to economic factors in historical analysis) . And there is that big chunk of neutral or enemy territory in between.
It could end up with two counterpoised empires - the (German) HRE and the (Spanish) Western Empire (whatever it ends up being called) . In such a scenario , it would suck to be France! The Spanish Road is too tenuous . I think that S-N-E has to either seize the old Burgundian territories or the two halves will sooner or later drift apart. If S-N-E does seize and hold Burgundy/Lorraine/Bar/Luxembourg etc then France is in a very bad way.
I think that, for the course of the joint monarchy between Mary and Philip, the par for the course administration of the Spanish dominions and the British isles would probably persist. I don't see any major overhaul occurring during their lifetimes/reigns. However, under the rule of a single King, especially a King with the right kind of ambitions, he'd probably start encouraging the inter-regional marriages and land grants as mentioned previously. The process of unification, if that's what "Henry" wants, is going to have to be carefully managed, but pursued actively. He'll need the right allies, and he'll have to be a shrewd politician to pull it off.
jotabe1789: "I agree with the problems of an Italian city, it's terribly located.
Having said that: Yes! Brussels! i love the idea.
The problem is that the united provinces might be a bit too small. Could do with a bit of ampliation. Like southwards. The French have a lot of extra room they aren't using, anyway."
I see this as a problem too... although I had mentioned the idea of Milan, as a previous Roman Imperial capitol in the west and already under Spanish control, is nowhere near central enough to make an effective nerve center for this budding empire. Brussels does seem like a likely candidate, though I'd love to hear more about it.
Also, you mentioned France... one serious possibility I see happening here is a future "Henry" getting an itch to invade France, perhaps under the pretense of putting down Protestants, but I see him as either wanting the French throne for himself (like every English king has since Edward III), or to take back (Aquitaine, Burgundy, etc...) some juicy French real estate and try to stitch his patchwork Empire together a little more cohesively.
Might France work a little harder to try and get the Turks to get involved here? With so few allies nearby and completely surrounded by enemies, I see the French getting a little desperate, and pushing a little harder to get into the Ottomans' good graces.
JedidiahStott: "In this scenario, England remains Catholic (of some sort). Scotland also. Spain is of course Catholic. The butterfly effects on the Low Country are hard to predict, but it seems not unreasonable that it may remain Catholic also, in the absence of English support (and, indeed, perhaps, England as a base to suppress any Protestant rebellion.)
Now, if we add that to the OTL successes of the Counter-Reformation, may we ask "Does Protestantism survive at all ?" . The Protestant base seems to be reduced to some of the Rhineland German states, and isolated enclaves in such places as Hungary and Savoy. And Brandenburg, I guess.And, with a Catholic Low Country so close to hand, would the Protestant Rhine states survive?"
It may be possible that ITTL, Protestantism may eventually be relegated to the status of a relatively obscure Christian offshoot, relegated to the status of Christian Gnosticism and Arianism.
I don't know if that's the most likely outcome... it seems that it had a fair amount of support in several HRE provinces and in the Nordic regions. It could end up being a primarily Germanic branch of Christianity. Or, it may even end up a Germanic national church, like Anglicism, the Armenian Apostolic Church, or the Coptic Church.