The code may take a couple of hundred miles or 20 engine starts before it comes back. Yes, there is a MAF between the air cleaner and the throttle body. But I suggest NOT cleaning it unless you get a MAP specific error.

A cracked manifold, exhaust or intake, could be a problem. But I suggest that the dealer fibbed. After 70K miles, the crack should have gotten large enough to be noticeable and annoying. Most shops had very little knowledge of 850's back then.

An O2 sensor at 130K miles is quite old and lived a good life. It may be time to retire it?

I would still take a good look at ALL of the vacuum lines for breaks, cracks, and loose fittings. Especially the 'hidden' elbow by cylinder #1 - a pain to inspect or replace. This vacuum line goes to the PCV valve by the throttle body.
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Keeping it running is better than buying new