Thatcher was never going to do better than Major was: her personal popularity ratings were through the floor before she resigned and I can't imagine any way that she'd have been able to turn it around: while going into 1992 despite dissatisfaction with the government Major generally had positive popularity ratings while Kinnock didn't. I just quickly looked it up and in the October 1990 Gallop poll (the last available before Thatcher resigned) her popularity rating was -31.7% (31.5% "good job"; 63.2% "bad job") while in the November one a month later Major's popularity was +30.6% with the numbers almost reversed and he had positive popularities right through to the 1992 General Election (although it was declining gradually and by 1993 he'd be in the toilet) and the public opinion polls never had him in a similar position to Thatcher - indeed the polls had 1992 really quite close, there's this misplaced impression that all of the polls were showing a Labour majority when they weren't.
She would be going into a fourth straight general election and no one has ever won four in a row: she had the stench of the poll tax and all of the other really unpopular government policies that her government had done which Major was a little disconnected from. I'm pretty damn sure that Labour would have won (although probably with a small majority on a lower turnout than the real 1992 election, the Lib Dems would have probably done much better in their target seats) although it wouldn't have been anywhere near 1997 levels - a small working majority most likely.
I agree with you on everything you said here: Thatcher was never going to win in 1992, and that's basically why she got ousted in 1990, because her own MPs knew that too. John Major was much more popular than Margaret Thatcher in 1992 and even he only won a 12 seat majority or something like that (336/646 seats if I'm not mistaken), and that was with the Tories winning the largest amount of votes ever received by a party. But that being said Labour wouldn't have won a large majority against the Tories and I think it's possible the Conservative Party could have been back in power five years later (because I suspect Kinnock wouldn't have handled Europe or Black Wednesday any better).
