The rule is quite clear of who would be eligible at this point in a brokered convention. Trump is the only one, and it looks like Cruz could also qualify with them being the two eligible candidates. As the rule states now, delegates could vote for either Trump or Cruz in that case.
That's nonsense. A brokered convention could, in theory, nominate anyone they want. Even if 40B were somehow enforced and Trump were the only one to be on the ballot, those opposed to him could abstain
ad infinitum and prevent him from getting the nomination. Unlike a primary, "none of the above" counts in a convention, and there's no removing that choice from the ballot. He must get a majority of delegates, and if the RNC cannot get a candidate with a majority of delegates, it HAS to eventually come to a consensus on another candidate, which has (in the past) taken dozens of, if not over a hundred, ballots and yes, it would mean throwing out any artificial restrictions on nominating a candidate.
Do you think the RNC is going to say "Well, we've taken the ballot, and Trump's facing too much opposition to get the majority. Not enough delegates are willing to switch. Shouldn't we consider another name?" "OH NO! Rule 40B! Trump's the only candidate allowed! We keep voting until he gets the majority!"
Actually, given Trump's love of brute force, that would be totally what Trump and his delegates would insist upon, and it would make a total mockery of what's left of the Republican Party (as if it hasn't made a total mockery of itself yet).