I've been getting the same lifespan that I get with the engines, close to 300K miles -- actually, I probably could get more out the engine and transmission, but the darn bodies rust away (I live in the northeast) too soon.
[This is with normally aspirated engines -- I've never owned a turbo -- which is, I guess, less stressful than a turbo even with a beefed up AW71. But I really can't comment authoritatively on AW lifespans with a turbo.]

The only thing that does go wrong with the transmissions, almost predictably, are the seals -- usually, I get a leak (often the tailshaft seal) and have to get that fixed at about 150-200K miles or so -- not unreasonable, IMHO.

Just change the fluid -- don't bother dropping the pan, as the screen is always clean (I've stopped bothering) -- regularly (I do it every 40-50K miles, or about every third year) and, for the past ten years, I've been using and I happily recommend, AmsOil's synthetic ATF. Use the IPD method -- no need to drain or drop the pan! Over the past ten years, I've owned five 240s (I've owned more 240s, but I only started using AmsOil ATF ten years ago) and still have three of them (the other two were junked only because of too much rust), and never had a transmission problem.
[Actually, I've used AmsOil synthetic engine oil for about 25 years -- maybe that's why the engines last so long -- but that's another story.]

As for your shift points, it's my best guess that your shift cable needs adjustment or replacement. No reason to go inside the transmission, unless that doesn't help. A few pieces in the sump are no special reason -- who knows how long they've been laying there. Only consider rebuilding or replacing the tranny if the flush (with synthetic) and shift cable repairs don't help. But I'm pretty sure they will.

As for your friend's mechanic, I would be suspicious. An '88 transmission is just far too young (unless the car has been abused, e.g., no fluid flushes) to need to be rebuilt or replaced.

Good luck.