im curious but why bother fixing a leak if you want to bypass the a/c compressor anyway?
anyway to answer your questions..
you cant fix the leak until the a/c system is empty of gas which it probably will be by now,get the o-ring from your dealer or an a/c shop.
volvo do make a serp belt for the non a/c cars so you could use that... but if you dont want the compressor to work then just disconnect the electrical wire that goes to the compressor clutch then it will never engage. i would only do the belt method if there was a mechanical problem with the compressor pulley bearing.

steve.