You really need a grinder or cut off wheel to grind the heads off of the bolts. Next remove the DP, grind the bolts flat on the rear flange and then drill the rest out. Get new bolts with nuts and washers (stainless if you can). Using a saw instead of a grinder is possible but will be difficult.
Hopefully your flange will be okay and you won't have to deal with any of this mess but you can try to find a flanged pipe, it would certainly make things simpler but my guess is that it would have to be ordered or made. You might get lucky though and a local store might have one. FYI alot of people will say female ends should always be the "flow in" side not "out" to reduce turbulence, but it shouldn't make a noticable difference.
Rear o2 just senses if the cat is working, it has no effect on how the car runs (unless it goes into a diagnostic mode, which is rare). If you don't want to run one, and don't want to throw a code then you need to have a PCM tune to remove the code. Some models can run an o2 simulator that will solve the problem. If you just want to leave it unplugged the only effect is a service light.
