oil cant mix into the coolant, but coolant can get into the oil. the brown coolant was old dex coolant. flushing it was a good thing to do.

if the o2 was causing the car run rich thats what clogged up the cat.

a walker cat would be best to use, something thats not hi flow, as those cats set off the o2 code, run rich again, 01 is a messed up year for the rear o2.