So, like it says in the title; what happens if Thatcher holds on to the leadership in 1990, what happens next? Particularly in the election of 1992?
Labour landslide in 91/92 similar to 1997.
I doubt it would be a landslide and it would certainly not be on the scale of '97: a '97 scale result for Labour is ASB under Kinnock even if Thatcher started eating babies on live TV. It would be a case of people turning against the Tories without any real enthusiasm for Labour. Like the 2010 election in reverse, except the Lib Dems' support base was much smaller in 91/92 so Labour will get a majority rather than a hung parliament--but it won't be a big majority.