My personal belief is that with the 1987 PM elections on the horizon, the combination of Thatcher's death and a bloody situation in Northern Ireland would have led to at the very least a hung election and the possibility of a Labour-SDP-Liberal Coalition,
...and you're making the same mistake. If Thatcher had been murdered by the IRA shortly before a General Election the result would have been the biggest landslide since 1931. And, frankly, no GFA a decade later - low level civil war would be a feature of life in Northern Ireland
now.
You should note the sort of policies towards Northern Ireland that the Callaghan government adopted. There would have been no chance of the Northern Ireland brief going to a pro-Sinn Fein figure and not even much chance of it going to someone associated with the SDLP (which was routinely done when Labour was in opposition, but not when it was in government). Position would probably have gone to someone who's career was in decline so as to kill it off forever, but also would have done what the civil servants wanted - Roy Mason redux, in other words. This is even more true if (somehow) a Labour-led government took office after Thatcher getting blown up.
Don't confuse that with support for the IRA or a military solution to the troubles, whatever you do. Most people didn't care about Northern Ireland - it was regarded as a strange mad land across the sea where people kept on re-fighting wars that happened centuries ago, all hated each other, blew each other up and wore silly little bowler hats. Probably the views of the SDLP would be closest to those of most people on this side of the Irish Sea.