Yes you can get a false positive reading. In my experience on working with cars for the last 25 years that happens all the time.

Seems like they did not solve the original problem from what you are saying.

I would tell them that you took it to another mechanic before and after and that the trouble codes were the same P0410 and see what they say.

Engine oil entering the harness? sounds like a fire safety hazard to me-check for any recalls and maybe call TSB. If oil that is hot is getting on a wire affiliated with emission/fuel injection operations it can cause a signal to O2 sensor to get misconstrued and cause a light to come on.

Would probably need a little more info on exactly what hese parts are that they replaced.

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(1) '89 190E 2.6 Schwartze
(2) '89 190E 2.6 Schwartze