Trump is running as a populist in the tradition of William Jennings Bryan. Very similar to how Perry ran successfully for Governor of Texas repeatedly (although Trump is more articulate).
Not a Trump supporter. Doesn't make my Top 100 list of qualified candidates. If it is choice between him and a Demonrat, I'll reluctantly cast my vote for Trump as a giant middle finger to the establishment.
As for Rush Limbaugh, what he's doing is promoting conservative principles by referring to or inviting as guests for interviews those presidential candidates who have espoused conservative principles. Note that Limbaugh is not endorsing anyone. However, if a candidate does something newsworthy for conservatism (e.g. on immigration), that's likely to get Rush's attention and a favorable mention at a minimum.
The problem with that, of course, is if Trump is playing everyone as a fool and plans to veer left if nominated.
Right now, that's the $64,000 question. I'd say it's 50/50 he pulls a Perry and stays with conservatism as a populist (but not out of principle).
The safer bet for conservatives would be Cruz. However, unless Cruz's ground game delivers in Iowa and the SEC Primary, there's no road to the nomination for him. Not a big fan of Cruz because of H-1B visas, the Corker Amendment, and the TPP/TPA fiasco. However, he's looking good in comparison to the others running.
You don't know that. Since his mantra is to make America Great Again, why would he keep steering the ship in the same direction?
He wants to be a successful president in the mold of Ronald Reagan. Why would he stay on course to be the worst president in the mold of Obama?
If Trump gets the nomination he will have a mandate to turn the country around.