Yes, the GOP will likely divide over Trump. It is already happening. There are Republicans who support Trump whole-heartedly, Republicans who oppose Trump completely, and those who support the GOP at all costs. Unfortunately, all groups involved feel very strongly and there is not likely to be a compromise. Trump is only a symbol of a very real problem.
What Harry Reid says isn't going to make a difference to Republicans, but Trump's ability to constantly open his mouth and insert his foot is embarrassing to the party. He is only going to get worse as the real pressure gets put on closer to the elections, and more Republicans are going to step back from him in hopes of keeping the Senate and House.
Although some members of the GOP are continuing to toe the party line, there is already a divide within the party itself. The "party of Lincoln" and the compassionate conservatives are not in the majority and despite their own research indicating that they need to be more inclusive and appeal to women, that's not happening. There are several prominent Republicans who have stated they won't vote for Trump (Romney, etc) and none of the former Rep presidents even came to the convention. I think it's pretty clear there is already a divide.
No, the GOP will stay intact. What we see and hear now is superficial posturing intended to score political points. If the GOP would indeed split, these people would lose their own power base and almost certainly put an end to their political careers. Facing the prospect of having to do honest work for a living, it is fairly certain their blustering will stay at exactly that.