The issue with using crusaders to keep Egypt Coptic is that the crusaders despised Eastern Christians. The Crusader States in the Levant mistreated non-Roman Catholics and Constantinople was sacked by Western Crusaders. So I'd say that you'd need an earlier POD to keep Egypt majority Christian
I dunno, I think the best approach is a combination - the Coptic Revolt rising up and taking power as the 5th Crusade arrives in Egypt - and have the 5th Crusade fail as per OTL - or at least have the Crusader Element retreat, whilst the Coptic Revolt obtains support from the Emperor.
Quick back of the napkin
0) Good robust revolutionary starts to make himself popular, and gets (surrupticious) backing from the Coptic Patriarch.
1) Crusade takes Damietta and kills Al-Adil
2) Seeing this as the right moment, Revolutionary starts the rebellion, with rapid success in Upper Egypt, whilst Al-Kamil is trying to deal with the Crusade
3) Crusader Army under Pelagius is surrounded and attacked by Al-Kamil
4) Revolutionary forces intervene and surround Al-Kamil, leaving him trapped between the Coptic peasants and the Crusaders. (VICTORY)
5) Pelagius and the Crusaders, grateful for their rescue, despite being rendered vastly weaker, agree to exchange Damietta for an alliance and logistical support/military support in Palestine.
6) King Revolutionary the Nth rules as King of Egypt, allied with the Kingdom of Jerusalem.
I'm not quite sure how the Revolution takes over the various cities, perhaps simply because they are able to be inside the cities and overwhelm the guards time and time again (or completely blockade them from any merchant traffic).
After a while, I can see Egypt being an intermediary between a restored Roman Empire from the Nicaean Empire, and the Kingdom of Jerusalem - essentially negotiating agreed upon borders and domains - with the Romans having Europe and Anatolia, Jerusalem has Syria and Palestine, and Egypt maintains Egypt - and agreeing to defend one another. Leading to the unusual situation of a Coptic Egypt, Catholic Jerusalem, and Orthodox Rhomanion dependant on each other for defending against Persia. (Although, admittedly, Catholic Jerusalem would probably get the worst of it with each invasion).
I could even see Egypt assisting against the Sultanate of Rum in Anatolia when the time comes. Essentially opening a southern Anatolian front, whilst the Romans focus on the North.