it sounds like you have a mess you need to prioritize things your wiring sounds like a mess make sure all your ground wires are hooked up. You probably have a bad O2 sensor it's not your fault w/ the oil in the connector that won't do anything or the oil on it you already messed up your O2 sensor by striping it so try to fix the mil eliminator remember the mil eliminator won't fix bad 02 sensors and your ground wire problem clear the code see if it still comes up if it does it's time for a new 02 sensor you stripped the bolt so now u can't use a wrench or 02 sensor socket so what u have to do is
A. take a vice grip get it real tight and put it around the bolt and turn it counterclock wise.
B. get a punch not the one w/ a round end the one with a sharp straight screwdriver like end and it w/ a hammer counterclockwise.
C. take it to a shop w/ a torch and they'll take a vice grip to it and get it out in lk 5 minutes but you're going to have to cough up at least 30 maybe even 60 cuz most shops charge 60 to put an 02 sensor in
now after that your check engine light should be fixed then go ahead with the exhaust leak if the exhaust leak is b4 the 02 sensor it might cause a CEL to come on to so if u go through all that and it don't work try and fix the exhaust
it sounds like you're new to cars its ok we all were at one time u have to mess up somewhere and learn somewhere too but go through everything you did with the exhaust check the flange see if that is on correctly make sure when you tighten the flange that you go back and forth with the two bolts not just one bolt at a time make sure they're equal i messed up a couple times doing that the custom exhaust is probably were the leak is at so while the car is running feel around for air exiting before the muffler thats where the exhaust leak is at then fix accordingly long posts take long answers i got nothing else to do in college all day (homework):hsdunno: so it is cool to put up long posts.