I once read what I thought was an interesting take, at least as to #3. At the time of the early crusades, both Europe and the Middle East were feudal. Notwithstanding modern Islamic mythology, European feudal lords took the place of Muslim feudal lords. The local Arabs were not entirely happy with their Seljuk conquerers and didn’t resist with the strength they might have. Neither Arab nor (some) Seljuk nor Christian feudal leaders had any motivation to really oppress the ruled (who would till the fields and press the grapes?) who consisted in Muslims, Jews and Christians. The ordinary people didn’t much care who the feudal rulers were, as long as they played their role decently and competently.
In point of fact, on a feudal level, Muslim and Christian leaders and forces were often friendly, and often allies.
But feudalism, both Christian and Muslim, met a different kind of force. Turkic Mamluks took over Egypt from the relatively friendly Arab ruler there. At about the same time, the Mongols invaded from the East. For a time, Christians and Arabs began to form an alliance to fight the Mongols. The Mamluks, who wanted to rule the whole, wouldn’t have it and drove the Christians out of Palestine. Allied, then, with Arab forces, the Mamluks defeated the Mongols who, however, had pretty much wiped out Islamic civilization everywhere they went.
Later, the Ottomans took over Asia Minor and were a very powerful group. Then Timur and his Central Asians swept in and destroyed what was left of the Middle East. Timur nearly defeated the Ottomans, but didn’t quite get it done. The Ottomans ultimately took over the whole of Middle East except for Persia and completed the devastation
The Middle East was, by then, utterly laid waste; not by Europeans but by Central Asians; Mamluks, Ottomans, Mongols, Tatars. It happened because elite feudal forces, both Christian and Arab Muslim, could not deal with the massive “total war” forces of the Central Asians, being hugely outnumbered.
As it happened, though, the Ottoman/Timur war played a big role in letting Byzantium dodder along for awhile, as did the Crusades themselves. The Fourth Crusade was a bad chapter, but until then, Byzantium gained from them.
In my opinion, it is incorrect to claim that the Fourth Crusade fatally weakened Byzantium and caused its later fall to the Ottomans. The Franks who conquered Constantinople in the Fourth Crusade numberd about 10,000. The Ottoman army that eventually took Constantinople numbered about 200,000. If Constantinople was too weak to defend against 10,000, it would have had no chance against the final Ottoman assault either, and didn’t. By then, the Ottomans also had cannon, which the earlier Franks did not.
**