Donald Trump may now be the presumed Republican candidate, but the party convention is not scheduled to close until July 21, and there will not be a nominee until then. Two and a half months is an eternity in this 24/7 media environment. There have been 22 contested party conventions since 1876, one lasting 103 ballots. The fat lady has not sung.
The GOP has survived Richard Nixon, Herbert Hoover, Progressive Teddy, and George W. Bush—and it will survive Trump too.
The Republican Party is an independent association recognized by the Supreme Court as such, able to set its own rules and procedures. And its ultimate rule is that the delegates elected to its convention every four years are in charge, just as state electoral votes are in the general election.
Trump will undoubtedly have 1,237 delegates pledged to him beforehand but that must be confirmed by actual votes from delegates.