ALDA is off now. And the power instantly returned.
So if this disappearing power thing ever happens to you, the answer is "yes" it is possible for an ALDA to go phfpbbttt! overnight, all by itself. And when it does, you drive a sled the next day. This was ALDA number 2 on this car... I think I'll just do without now. My overboost solenoid has already been bypassed for a long time... I'll (not) miss you ALDA!
Removing the ALDA addressed my problem. I might do another ALDA teardown to see if my theory of one ruptured aneroid capsule was right... the seal seems to be OK with a blow test. It holds "mouth" pressure even with me pushing the plunger up into the ALDA body. Up to a point.. if I push and wiggle a bit the ALDA will release the pressure
I also learned a few things about my turbo and wastgate in the process. The wastegate spring must be getting a little soft / opening too early because I could only pull around 0.7 bar (absolute manifold pressure) under full load. That's a bit on the so-so side for turbo pressure. At the altitude of where I live my gauge shows typically -0.1 bar absolute manifold with the car OFF. (So this means the turbo was providing about 0.8 bar boost maximum.)
With ALDA off and full pedal, I could blow a little black again, a good sign the car was not starving for fuel
I installed a manual boost controller spliced into the wastegate line, and adjusted it to get about 0.9 bar absolute pressure from the turbo under full load (about 12.8 psi). She spools up a bit faster too, because the wastegate is held shut longer ...until something much closer to its pop pressure is reached.
The extra air sooner cleans up some black smoke at WOT! A manual boost controller ($20) is a great mod to go hand in hand with ALDA removal.
Ahhh... the driving experience with ALDA off, gives me all that power early in the pedal. I have to get used to that feeling again... very very addictive.
Right now I'm fish tailing up hills and squealing around corners again!
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