The spoiler label would stick quickly.

... the 'hypocrite' label.

If he's the maverick everyone thinks he is, he'll abandon the political hacks and get back to the business of defeating the republican nominee.

It's easy to get keyed up on the campaign trail. Emotions run high, and no doubt, his well-paid campaign managers are encouraging him to keep going after Clinton as hard as he can. He probably needs a breather and chance to decide what it is he wants to accomplish at this point, since the nomination is out of any practical reach

In Bernie logic, rules that have been in place for decades (closed primaries) are voter disenfranchisement, while subverting the will of DEMOCRATIC VOTERS is not.

Sure, campaign managers will try to flip all the delegates they can, and if they can't, they're still not going to admit defeat before their candidates concede. It's not overturning the will of the voters (depending on the leverage used, of course)...but asking a campaign manager to concede in advance is definitely stupid.

Let's calm down and let voters and the candidates negotiate.

And the conventions are just free press for the party/nominee

...but never down to just one faction of convention-goers. Even delegates may hold out a while, though this isn't often visible to those of us outside. Eventually, most at the convention will plaster on smiles and pretend to be 100% for the nominee.

And eventually, it's more or less okay.

...some leadership in that direction from the top would help immensely.

...they're outta here!