I think anyone who bothers to go vote in a Republican primary is extremely likely to vote for the Republican candidate, whoever it is. If Trump loses the primary, I don't think you'll lose a meaningful amount of Republicans.¹
I do think Trump has significantly more support from independents and Democrats than a typical Republican. This makes sense because, as mentioned, he's not really about a political philosophy. However, I don't think it would be a net loss to the party. As many indies/Dems as they would gain, I think they'd turn off even more than that. He's just very polarizing in general.
I think that if Rubio can avoid early elimination, he should be able to win it. Keep in mind that the party leaders despise Cruz as well. I think they will only be willing to put up with Cruz if they're forced to choose between either him or Trump.
I would not rule out Christie, either. He is not polling well overall right now, but you can basically throw that out the window once the votes actually start. It's easy to envision a scenario where Christie does really well in New Hampshire, and then momentum kicks in. He's certainly one of the Republicans' most "electable" candidates, IMO. And although crazy things do tend to happen early in the Republican campaign (especially in Iowa), the electorate always seems to come around to who can actually win.
Bush doesn't have an audience, IMO. If you want a moderate, there are better options; if you want someone electable, there are better options (for one thing, being named "Bush" really hurts one's electability.) I don't see a path for him at all.
¹ That is, assuming that Trump isn't still an option for them in the general election. I don't anticipate Trump running as an independent, because he can't realistically win that way and that is what he cares about.