PIRA generally went out of their way to avoid causing civilian casualties.
Err. No. The provos were pretty bad in this respect, more extreme republicans were even worse. Loyalists the same. Civilians always bore the brunt during the Troubles.
This question is hard to answer because it relies on interpeting psychology of the people involved - if Belfast had failed, would there have been an intensification of the violence, or would there have been a renewed determination to bring forward an agreement?
IMO if the talks had collapsed, then you would have had it pretty bad for about a year or two, before everyone rallied the troops and sat down again. I'm simplifying, but by the 1990s the problem, in terms of securing a political settlement, was increasingly not republican politicians, but unionist hardliners - republicans had began to change how they percieved the conflict from the early nineties onwards, and became increasingly receptive to
some kind of political settlement. So I would hazard that by 9/11 we would have already have had an agreement, or be on the way to one. In the short term, though, I think a failure of Good Friday would have lead to a dip/rise in the violence in the immediate short-term.