nikibr

Okay so heres my story for which i really really need input. I have a 2001 gs 430 that i bought two years ago. Has about 70K miles on it right now. With CEL lit up, i went to autozone and ran a diagnostic which showed that it might be the front oxygen sensors. My car apparenty has two of these so bought both but were not OEM. My Mechanic replaced one side but couldnt replace the other saying that it was too tight to be removed and will have to be done by the dealer or a repair store. i also believe he didnt reset the computer by disconnecting the battery. However, after he changed one of the sensors, my car severely started to vibrate upon idling. Thinking that this might be because the other one isnt replaced yet, i went to a repair shop and they charged 700 dollars to replace the second sensor as they needed to remove the exhaust. The vibrations were still on and so was CEL. Showed it to another mechanic and he cleaned up the throttles and replaced the air filter. The vibrations are down a bit but not completely gone. Additionally while doing this, the mechanic noted that one of the cat is bad and might infact be a aftermarket part too. he also mentioned that the pick up of the car which is a V8 is not as much as it should be and therefore suggested to replace the two cats for a total of 1300 incl parts. He guaranteed that the car will be perfect after that.

is he right? could the vibrations be because of a bad cat? if yes why will they show up only after i changed the O2 sensor? Also if it is necc to change the cat, is it recommended to change both even if just one is bad?

Please help. I would truly appreciate it.