Trump actually acknowledged the issue the other day when he said that he believes the convention would give him trouble if he was even two votes short of a majority of the delegates. That’s not agreeing to such a thing, but it is acknowledging that the convention sees differently a candidate who has a majority and one who doesn’t.
That’s actually a sane way for a party to look at a primary. A plurality vote winner is not necessarily the best candidate depending on small their plurality is.
If Trump were to have 34%, Cruz 33%, and Rubio 33%, then Trump would have more people having voted for him than the other leading candidate, but in terms of republican voters across America, 67% would have NOT supported him.
It would only be sane to sit down and discuss the best course of action. I don’t think the right answer is to impose someone who ran even more weakly or who hasn’t run at all on the party (A Bush, A Romney, A Pence), but I do think that it calls on them to find some way to get as many of those votes on that ticket as possible.
They should take the VP selection away from the Front Runner. If his plurality is just 34%, he should also have to agree on appointed X person as SecDef or SecState. He hasn’t demonstrated sufficient support to be permitted to make such selections purely on his own strength of support.
These are all party rules and not constitutional issues, so they could be implemented fairly easily if so desired.
That would be illegal. It is a violation of federal law to promise a federal post to someone to help win an election, or, in this case, to secure the nomination for a federal office.