Few points you're missing...
Ken Clarke - zero support in the party.
Maybe not, but then neither did Churchill before Chamberlain's trip to Munich. Not that Ken Clarke is in the same league as Churchill; or likely to become short-term PM for that matter. As "Father of the House" - i.e. the oldest sitting MP (if only by dint of having taken his seat in the chamber about 20 minutes before Dennis Skinner) he does however command a great deal of cross-bench respect.
David Milliband - not an MP, and lost to Ed last time - no support in the party.
You assume that the next election is going to be another two-way fight between the Tories and Labour.
This whole Brexit shambles has created millions of disaffected voters. Come election time, these people are likely to turn on their sitting MPs and offer a vast voting base third parties are going to do their damndest to appeal to/exploit.
At the more - for want of a better word - "acceptable" end of the spectrum, you've got The Independent Group - who could easily parachute DM into a safe seat and run him as leader. It's happened before with various ministers and at least one PM (Alec Douglas Hume - although that's hardly a ringing endorsement of the practice).
There's also however many disaffected Labour MPs Tom Watson might be able to muster and take with him should he strike out on his own.
At the less palatable end of the spectrum are demagogues like Farage and his Brexit party. In the toxic sludge at the very bottom of the pond are so-called "populist" racist nutjobs like Tommy Robinson.
Chuckles - not a member of the Labour Party or indeed any party as yet.
Tom Watson - no support in the party -
See above comments re TIG.
I've said it before and I'll say it again. Cameron's stupidity in announcing a referendum before he and his aides carried out any kind of due diligence or put even the most basic checks and balances in place has created horrific social divisions beyond even Thatcher's wildest dreams.
The resultant fall out is going to poison the UK for years and ain't going to end well.