Right now the UK parliament are not going to vote for the deal. If Corbyn pivots and votes for the deal then he is going to be roasted – that pressure is not going down. Both sides of his support want him to whip against the deal, and there is very little cost to him of doing so, he can oppose the Tory Brexit, let them get the blame, and he doesn’t care much about the damage.
The EU will probably offer an extension to 22nd of May and that will cause a row today, Theresa May will probably be tempted to fling it back in their faces as that supports her “blame everyone else” narrative. If she does get an extension to May or end of June it will be contingent on the commons passing the deal – which they are almost certainly not going to do.
This means a possible last minute re-think, where the EU offers a long extension, with MEP elections and subtle suggestion that other democratic events in the UK might help matters. I don’t know if Theresa May would take up that offer, or even if Corbyn would make a statement saying he likes the offer – either way the EU can’t do more than offer it.
After that we are out. The withdrawal agreement is dead, and the legal basis for offering it is gone. I suspect it will be hastily redrafted to be something that can be offered under article 218 which is about relations with third countries – it would probably need to go to all member state parliaments rather than just signed off by leaders – I suspect getting the agreement of some other parliaments might be a struggle at that point.
We could in theory rejoin through article 49 process. There is no speed limit on that and no queue, but I think it would be rather painful. I suspect that taking a reheated withdrawal agreement through article 218 would be more probable.