I have heard many discussions on this topic but my main concern is if the man chooses to convert to Islam, although he now believes in Islam and follows it, his initial incentive or motivation was associated to pleasing his future wife, rather than because of belief. In this case is it still allowed for a Muslim woman to marry him? Please elaborate with proof if possible.
There is no dispute in Islam that a Muslim man can marry a Muslim woman.
In your case, he is a believing Muslim and she is a believing Muslim, no doubt that it is allowed.
It's permissible for him to marry her, even though his intention to convert to Islam in the first place, is for the sake of the marriage. This is one of the way Allah give "hidayah" (guide) to people; through marriage. Some people are challenged with this conversion thing when they want to get married to a Muslim, but they decided to end the relationship rather than converting. Wallahu 'alam.
Yea, if the christian man is reverted to Islam, then, I don't see any problem for a Muslim woman to marry him. So, She can marry him. But my opinion is that it'd be better if he reverted to islam for understanding Allaah and his oneness, not for marriage. But even if it is for marrying someone he loves, Allaah Subhanahu wata'ala may guide him.