Ok. Rubio is still losing. And his delegates are the difference between Trump and Cruz (acknowledging that it is unlikely all of them would have gone to Cruz).You're talking about a difference-of-differecnes of 23 delegates: 0.1% of all delegates.
First, what demos are you referencing? I think you're wrong about expectations, but I'm not sure which demos you're talking about.But the point is: Cruz badly underperformed in his likely-strongest states, and with his likely-strongest demographics. Rubio won Minnesota and performed well in Virginia, which are representative of his likely-strongest states and likely-strongest demographics.
Second, if Rubio "does well" and loses, he still loses.
Third, if Rubio wasn't in the race, where do you think those demos would go? Probably not to Trump. IMHO.
Generally, the first ballot goes to their candidate or the candidate endorsed by their candidate. After that, it is open season. That's why it is important to get the delegates/endorsements to get over the 50% mark on the first ballot.I suppose that pledged/bound delegates could go to whomever their candidate endorses. But I think it is more likely that, if their candidate is not qualified to be presented for the first ballot, then they are free to vote for whomever (i.e. they become unpledged/unbound).
Sometimes, the delegates can be "released" - either formally or informally - from their first-ballot obligations.