How do MPs force the long extension if the government doesn't want to entertain the idea?
They can't - May has secured a shorter extension from the EU than she wanted. She wanted June, they gave her May. We can't force the EU to give us longer, but naturally if there was a vote to remain they'd accept that straight away.
I agree with Andy's analysis, May's even lost her chief whip's confidence and he's being openly critical. That's about as bad as it gets for any PM (or should be).
I think the MV3 will have to happen first, but a no confidence vote next week is a real possibility. May got 63% on December 12th - a lot has changed since then and it's hard to see how she could survive one. Then the Commons could take control and come up with something sensible by April 12th.