The Tories actually had a clear lead in most of the polls from beginning of the Major premiership until the end of the Gulf War.
Possibly - I'm going from memory, but I have a spreadsheet and graphs at home. I know even with Major, Labour went back into the lead soon after the Gulf War and kept it for most of 1991.
But yeah I think Heseltine would have gotten them about 5 extra points initially; he would have been seen as a bigger change than Major and would have appealed more to Lib Dem voters.
I think initially might be an important qualification - I don't believe Heseltine would have campaigned as well as Major did in 1992, so I'm not sure he could have sustained the advantage.
As for a 1988 election, I don't see many plausible reasons for Thatcher panning out that parliament to it's very end. But possibly a poorer showing in the May '87 local elections?
Yeah, she took the '83 and '87 locals as weathervanes and then decided wether or not to call a general. There were a couple of scandals that broke later in 1987, that might have given her cold feet if they'd broken before May and hurt Tory support.