Stumped - No cabin heat, but...

No Doc, it's interweb guessing, you done good. We still don't know, continue to try helping.
Couple things strike me, despite what sounds like messed up coolant system you're still able to drive to and from work. Not reading anything about loosing coolant, swapping fluids. I've never experienced head gasket failure in a TDI, but most cars, if you open the coolant system (overflow) and rev the motor, it will blow out coolant.
I also believe the no cabin heat is just a system of whatever issue we have. But rule it out, uncouple the 2 hoses to the core and flush.

Going to tear into it tomorrow or Wednesday, developed another leak, am assuming it's the rad based on location. Found an entire from clip used with fans for $50, will update what I find.

Got into the car last night, took a hose off the heater core, easily blow air through it, no resistance to speak of. Took off my zero start as well, none there either, check valve had been removed. I didn't mention that I used a thermostat off an old engine I had laying around, out of my old car, but was fine when last used. Since then, wondered if it may be bad, so since I had it apart again, I took all 3 thermostats and heated in hot water, one opened much sooner and wider than the other 2, one hardly opened at all. Did find another leaking hose from the coolant tank to the block, so replaced it entirely, was pretty spongy. Replaced the lower rad hose as well, same issue. Going out now to install the thermostat, had pinched the last o-ring, tried but it did leak. Picked up a new one today at the dealer, and it is much different. No longer round, more flat and ribbon like. Swears it's the same part number, so will give it a shot, said it might be an updated part, though in the 10 years I've been working on these cars, never seen another like it. Anyone see a thermostat gasket/o-ring like this before?

No but it looks like the QA seals with the nodules...I don't know why they'd have those because there's not a groove for it to sit in unless it's more universal now and the newer models have a groove????

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Wrong one, fully 1/4" bigger on either side of the flange. It does look like one on the pump with the notches. Bring it back and see if they have the right ones, bringing the old one too to show them the difference. Knew it didn't look right....

Got a gasket from partsource tonight, got it installed. Seems to be working as intended. Have heat, temp stays right around 90. Only thing has me worried a bit is the lower rad hose was still cool to the touch after a 45 min drive, and the hose has a lot of pressure, which does release while opening the rad reservoir cap. Any idea how much pressure is normal for that lower line?

There shouldn't be any differential pressure, it's all under the same pressure. The lower hose will be cooler than the upper because it's cooling as it passes through the radiator

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I think we're good then, a few days will let me know for sure. Thanks again for the help.