'No serious GOP politician is going to damage themselves with by association with Trump. '
No, they are already starting to come round because they realise that Trump has cross-party appeal and the majority of the Republican base will stand for no one but Trump, no more Establishment puppets. They have had enough.
Bob Dole came out yesterday and said that Cruz would be "cataclysmic" and Trump is better. The Establishment have no choice, Trump has beaten them, they will have to back Trump in the end.
'it wouldn't be the highly ambitious and image conscious (though admittedly pretty thick) Rubio.'
All of the politically correct Establishment ones will tell Rubio to work wth Trump thinking that they will outsmart Trump in the end.
'Candidates very rarely pick failed fellow candidates as VP nominees beacuse (a) there's normally a lot of bad blood between them'
Yes, but unlike Cruz, Rubio has not yet really attacked Trump.
'they wouldn't pick the only other leading fellow candidate with a problematic religious story.'
The Establishment have used "religion" as a trick to fool their base and intentionally lose and give in to the Democrats on every major issue. What the Establishment have discovered in this cycle is that the base doesn't care, Cruz will lose and they all want Trump because the economy trumps religion.
'they are by definition failures'
Everyone is a failure against Trump, but Rubio and Cruz are exceptionally clever people even though Rubio is Establishment. They aren't failures and one day everybody says that Rubio will become President. It took Reagan three attempts to become President. None of them are failures.
'Then, obviously, there's the fact that Trump isn't going to get the nomination.'
Yes, the Establishment may still be able to stop him but it will be one hell of a fight.