Interesting idea. I want to pick up a couple of points.
I don't think Wilson would be "done" just because the vote on the Queen's Speech failed. All such a failure proves is that there's no majority in the House and there needs to be another election if the government wants to pass new legislation - this hasn't happened since the 1920s (though 1950 was close), but was a fairly regular occurence before that. Wilson wasn't done after Labour lost the election in 1970, and Labour doesn't have any history of ditching leaders. There might be a leadership challenge but it's still going to be very risky for the challenger (too risky for Callaghan), and Wilson is still going to be the favourite not least because there's no obviously viable alternative, no candidate who seems capable of both keeping the party together and broadening its electoral appeal. That being said, he might decide to go now rather than fight on - but it is his choice and I suspect he won't do that until there's a viable successor.
Secondly, Michael Foot wasn't new to the frontbench in 1974 - he'd been in the Shadow Cabinet since 1970, and would have been in the Cabinet in the 60s were it not for his insistence on democracy over patronage (he didn't want to be appointed to the front bench and be beholden to the leader, he wanted to wait until he could run successfully in a Shadow Cabinet election). And if there were a leadership election and he did conclude that he was too new to run, then the leadership of the left would almost certainly rest with Barbara Castle. She almost certainly wouldn't do as well as Foot but I'm certain she'd go for it if he wasn't; it would not be a straight battle of the right.
(Also, it's the second time Wilson has become PM, not the third.)