What do you mean by "steals the nomination"? The Republican Party has rules some of which have been in effect for 160 years about how conventions are conducted. It takes a majority vote of the delegates to win the nomination. If no candidate has a majority of delegates locked up prior to the convention, there could be a contested convention. It's no different than if a candidate does not get a majority in a primary and there is a runoff election to decide who will be the nominee. Coming in first without a majority is meaningless.
If a candidate participated in a state primary and does not get that party's nomination, that candidate is not allowed to run as an indpendent in that state's general election in the fall or as another party's candidate either. That's long standing election law in most states.
In my state of Texas, not only can someone who participated as a candidate for office in a primary not appear as an independent candidate or candidate of any other party, but anyone who voted in a partisan primary is not allowed to sign a petition to get an independent candidate on the general election ballot. An independent has just a few weeks to apply as an independent candidate for an office, but must get the required number of signatures from registered voters who did not vote in a partisan primary in that election cycle.