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I had a career in HVAC for a while, and I'll apologize for not having read all the thread here .. but I have a couple questions.
1.) Was the AC working when you drove it home from the shop?
2.) Has the disiduous earth filter ever been replaced in your log (aka dryer) ?
3.) Has any other work been done on your AC ever?

The BMW AC is one of the simplest ever.

So you want to eliminate obvious electrical, confirm there IS a leak , and determine if the pressure sensor is allowing the compressor to engage.

So I would do a reality check before anything else. (because I don't know anything about the shop)

Start car, turn on ac to full.
1.) locate the compressor (you can look down at it from the passenger front). Observe if both pulley and center of pulley are spinning.
-if both are spinning you don't have much of a leak. There's about 20 oz of gas there. Its gone fast if you have even a small leak. The center of the compressor won't engage (turn) if the pressure is low, or if the fuse is popped even when you have the AC "on" . We can be fairly sure they checked the fuse but if the engages you have both electric AND gas leak eliminated.

So what else is there?
- a moisture / dryer problem
This happens when moisture finds its way in to the system. It freezes in the expansion valve and won't let freon flow.
The job of the dryer is to catch & retain that moisture before it gets that far. Your car is aging gracefully, but we all forget there's a dryer in there, with 2 O rings for the hoses that eventually leak. , its in front under the passenger fender if I recall).

- non functional Expansion valve

I would remove the shraeder cover and very briefly push on the tripper (1.4 second) Did you hear gas come out?
they may have told you .. but you didn't mention if any gas was actually gone from the prior day's fill. Maybe if u r friendly with them I would ask.

I don't know if I helped .. but I hope so.

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