Some of those ideas are rather silly at this point, like EGW's claim that eeevvilll Rome is planning a vast end times persecution of everyone who worships on Saturday. Most Catholic parishes now have Sunday-anticipated Masses on Saturday night. I guess evil Rome is going to persecute ... herself?
Authentic Catholicism can "unite" with Islam only when Islam rejects their many errors. When we have an "Islam" that can say the Nicene Creed and believe it, then we can talk about "uniting" Catholicism and Islam, within one Catholic Church. Of course, they won't be Muslims anymore, when that happens.
As to what apostate Catholics may do in the future, most of them seem to obey Pope Zeitgeist. Maybe after they get bored with "ordaining" women and "marrying" homosexuals, they'll embrace Islam. Anything but actually embracing the truth ... see Lewis's Great Divorce in the scene with the Anglican prelate.
I always maintain we will all be surprised. Assuming that the SDA understanding and interpretation is correct, I will also be surprised as to how it all will play out.
I believe these things will happen, but I fully expect the manner of how it plays out will surprise everyone of us.
And as I said, it’s the devout and real Catholics that is who the Bible is condemning.
Anybody who is a real follower of God, regardless of their denominations will be called out by God just shortly before it all happens.