May couldn't be voted out as leader cos she won the confidence of tory mps and so couldn't be challenged for 12 months.
Technically, but the 1922 can amend the rulebook as it see's fit, so that it isnt worth the paper that it is written on. If there's a majority to remove May, there's a majority to make the rule change. Even if there isnt, she cant credibly continue if a large minority are against her.
In terms of a deal passing through parliament with the 2017 mps, well it happened otl with Johnson's deal despite corbyn whipping against it. If instead May convinces corbyn to allow abstaining or make it a free vote then yeah the deal is going to be passed.
Possibly, but Johnson succeeded because he was able to combine the support of the more radical sections within his parliamentary party with those who just wanted a deal at any cost by that point. In this scenario, the deal is going to have to pass with the opposition of both second referendum supporters and hard Brexiteers, so the calculation is fundamentally different.
Also, if the deal is going to be passed, a Labour abstention isn't going to cut it. 118 Tory MPs rebelled against the government on the first vote on the WA. Realistically, any agreement with Labour is going to produce a softer exit than the one initially proposed by May, so any rebellion against it is going to be just as big, if not bigger. Once you take into account the various minor Remainer parties, the DUP and the sizeable number of Labour MPs who will vote against regardless of what Corbyn tells them to do, you're looking at a loss of at least fifty.
If a compromise deal was going to be passed, it would require Labour to explicitly whip in favour of voting with the government.
The DNP said if that happened, they'd then withdraw their support to the tories which would like you say lead to an election but I don't know how much I trust that given OTL. And if Corbyn whipping to abstain on the vote one of the many times the deal was put to parliament, would lead to the government collapsing and a new election against May well it's difficult to see why he didn't do that then.
Such a scenario might well have been Corbyn was hoping for when he engaged in the cross party talks, but the problem with this in practice is that he also needed to appease his own side at the same time, first by securing enough concessions from May to bring enough of the PLP with him to pass the deal, and later by avoiding being seen as enabling Brexit by Labour voters who were flirting with the Lib Dems.
May was never going to offer him those concessions, because that would lead to her haemorrhaging support among her own MPs and voters, and being a human being who was herself capable of rational independent thought, she wasn't going to try and pass a deal that would result in the collapse of her government and in all probability her party as well.