[Legate of Amon Lanc]

I think the answer can be put from several directions, also depending on the way you look at it.

A) Manwë would simply never "fall to the dark side", because that's the way we feel it from the story. I think most, if not all the readers have this feeling. Otherwise, Manwë knows lot of Ilúvatar's mind... maybe even for example if there is any deeper reason behind the drowning of Númenor he may actually understand why the innocents have died... etc...

B) (Is rather a continuing of the first, but I am putting this apart to emphasise the different "level" of it.) Manwë could eventually doubt Ilúvatar sometimes, but his faith in Ilúvatar's doing being good would ultimately convince him to stay true to what Ilúvatar set before him.

C) (This could also go together with both those named above, it's just another way of seeing the things.) Manwë (and Valar) are simply "pre-destined" by Eru NOT to fall. Point. (This actually may be true in the deeper sense if you look at the story from the "outside" point of view, again, Tolkien wrote the characters in some way, so they are predestined by his inkpen, so to say. Of course Tolkien didn't write about what Manwë does in the Fourth Age, but if he continued to, the characters would be still predestined by his inkpen as he was in the ages before, where we knew Manwë as faithful to Eru. And again, one would probably assume he wouldn't differ from the Manwë we know.)

D) Yes, it's possible that Manwë would eventually fall. There's always a possibility.

I think there can't be said much more than this (basically).