You're way off Redtom - the major cost in NI is the security apparatus - a lasting peace in the province is not entirely impossible to achieve and i find it hard to believe this isnt possibe given the huge progress made recently. Once the peace is cemented there is HUGE tourist market waiting to be developed in the province - giving the UK a net gain in additional tax revenues etc.
As for the question posed -
1. Not justified. Regardless of whether you think violence etc is justified under such circumstances, the peaceful marching and demonstrating had the Protestant majority under a lot of pressure in the late 60's and early 70's and may have finally brought about the changes the Nationalist/Catholic minority were seeking.
The use of violence polarised the society even further and made a compromise impossible. That said, the IRA/INLA violence has successfully wrung concessions from the British over the years but it was the major military set-backs suffered by those groups in the 1980's (use of the "shoot-to-kill policy" by the SAS being a big part in those defeats) that let McGuinness and Adams to start trying move the IRA towards becoming a peaceful political movement.
As for the American Irish's support for the IRA - a more blind and pathetic collection of views is harder to imagine. Talk about living in a cartoon reality! Teenage boys from the Mid-West (who've never even been to Ireland and would have a hard time pointing to it on a map) with IRA tattoos on their arms - honestly!
2. The Protestants - yes, their treatment under the rule of the Republic would have been substantially worse. The (I suppose, nominally) Catholic citizens of that country had to fight long and hard to obtain the right to a divorse,abortion, contraception etc etc and to shake off the pervasive influence of the Church in the running of the country. Bad enough for Catholics, can you imagine how Protestants would have felt?
Perhaps more significantly, on a day to day basis, would have been the possibility of increased intimidation and violvence against them by Cathloic seeking "revenge" etc and the markedly worse economic conditions prevailing in the south which could only have worsened the north's unemployment/economy