On Mecca, it's more a matter of motivation than of feasibility.
I don't know which tribal confederacy hold sway over the region besides the
Sharifate, but the Levantine conquest of Syria and the neutering of Buri's threat with the TTL Second Crusade does leave only the Fatimids available for help, unless you make the Yemeni intervening, which could happen given it seems they are going to play a big role ITTL early on. Yet, as Egypt goes, Hejaz is virtually isolated and on its own. Before the Yemeni consolidate a state encompassing the whole of Hejaz, the Franks could have a window of opportunity to strike.
Their success would be contingent on securing a supply chain, which means already having penetrated deep into Hejaz, which I deem unlikely before the window of opportunity closes, or securing an outpost at Jeddah.
Else, they'll die of thirst before even making it to Mecca, or be trapped in their conquest (in which case the disastrous fate of the British army at
Gandamak during its retreat from Kabul could befall on them).
Plus, Hejaz is less of a desert than Nejd is, and a rather mountainous region.
So, on motivation, besides the religious anti muslim fanaticism, there is no much if anything to justify taking the town.
Strategically, Hejaz doesn't pose any threat to Jerusalem, unlike Egypt and Mesopotamia. Once Egypt is taken out, there is even less of a threat, and Yemen is too distant to be a serious threat even if a rising regional empire.
Demographically, the conquest of Syria and Egypt, not counting Palestine, would put a enormous Muslim population under Levantine rule. An attack or a sack of Mecca couldn't be possibly considered without the trouble it would bring among these subject populations. The risk of widespread rioting and revolts, and the cost of suppressing them, would far outweigh any value to draw from that operation.
Financially, it would endanger the enormous tax pool that are the pilgrims on their way to Mecca and Medina. That's way too lucrative a mana for Levantine lords to abandon it.
When you consider the norm among Levantine lords was tolerance out of pragmatism, the only serious motivation I cited, crusader fanaticism, is almost an impossibility.
Be it the Sharifate of Mecca or the Yemeni, there would be probably an agreement with the Jerusalemite (and given the fall of Syria and Hejaz complete isolation, it could happen soon enough in this TL), including some tribute to avoid raiding parties.
Edit: And before somebody jumps in, I only said influence their infantry composition, I really do not see Jersualem having a standing army like Byzantines do. This may be an AU, but they are still a feudal, decentralized realm, with nobles having quite a bit of influence and personal power, and they are most unlikely ones to allow the creation of a standing army. Though, as the time goes by, we could see an evolution of sorts, with "bastard feudalism" coming about, and system of liveries and contracts emerging, such as was seen in late Medieval England, before and during the Wars of the Roses.
It all depends on the fiscal apparatus. The reason why standing armies came so late is that it took a few centuries more for the fiscal apparatus to develop to such an extent that the state could support such a standing army. In France and England, you got to wait the financial pressures and strains of the Hundred Years War.
Hey so I just saw this post. Wouldn't Portugal or Spain want to eventually try to gain money for itself eventually, which explains the Age of Exploration pre-1453? Eventually IMO someone wants to cut the middleman (AKA the Byzantines) away in the West-East trade.
Though the ATL Euro-Levantine control of the trade routes to Indies delays significantly the discovery of Americas, it doesn't preclude the exploration of Africa, though it could still be slower.
If the
Great Bullion Famine of the 15th century still happens (perhaps earlier than IOTL as the open trade route to Indies would likely boost the outflow of silver and gold and worsen the trade balance), then an expansion into Africa to get gold would be a good motive enough. The wealth of the
Mali Empire for instance was renowned enough for Mansa Musa's pilgrimage to Mecca to make a lasting impression, for the man is considered as of today among the most wealthy men in all of history (about
$400 billions inflation-adjusted ahead of Rockefeller and Carnegie).
The difference here is that we get a different starting point in all likeliness.
First, we may have a more lasting and solid presence of Normans in Tunisia and Libya, strengthened by the fall of Egypt, then the delayed discovery of Americas would entice the Iberian kingdoms to pursue the reconquista into North Africa, just like they almost did IOTL.
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