Even 'forced' Christianity wouldn't do it if they really believe that in the end Allah will save them.
The exercise would be one of futility.
Just as most Christians can't be forced to be muslims. Most muslims will not be forced to be Christians.
It's better to let them stay bewildered and lost for as many generations as necessary then to create an even greater hatred of Christians, which would give them a stronger reason to hold to their faith and allow them to recover sooner.
As you say, the devastation must monumental and complete in order for them to even question Allah and his teachings.
Forced Christianity will, indeed, eliminate the threat of Islam for, at least, a very long time. In the second generation it will begin to seriously "take." The defeated moors will not, in the main, teach their religion to their children. They will retain it but will not be enthusiastic. Yes some secret groups will remain and willfaintly trouble the world for millennia, dort of like Rosicrucians. The Marranos of Spain and, derivatively New Mexico, still retain memories and customs of their Jewish ancestors, some even call themselves Jews, but the Religion is gone, only the customs remain.
A greater hatred of Christianity cannot be created. Only Satan himself has that. You seem to think of Mohammedans as essentially just like your next door neighbors, perhaps a little odd, but essentially rational. No. they are Mohammedans. I know something about that fatalism and how it works. You cannot simply beat the people down. You have to replace the operating system and reboot.