Silver_TT

When I needed mine I went with the Bosch reman and it was about $350. Also no experience with the cheap ones but I prefer to stay OEM

Learn something new every day.........
I just spoke with Sylvania and they told me the industry standard is to test 12V bulbs up to 13.5V (because of fluctuations in auto electrical system, etc). They said this often causes confusion because the information on the packaging and the product itself is inconsistent, but that the bulb is in fact a direct replacement and compatible in the stated vehicle.
I love this car so I adopt Jim Harbaugh's motto.... "stay paranoid" ;)

Problem was solved by replacing the engine purge fan. $90 all-in included shipping from Sunset -- who is awesome by the way: I ordered the fan (which is a special order part) on a Monday morning and it was delivered to me today, Saturday. Jesse and the others in Parts over there at Sunset are outstanding. Very much on the ball, I've done quite a bit of business with them over the years and they never let me down.
The fan part comes with the housing. Replacement is so simple I didn't even bother posting a picture of it. There are 3 bolts around the fan housing that hold it to the trunk lid. Move the light over to the new fan housing. Takes less than 5 minutes to replace. Popped in a new 15A fuse and it's holding just fine even with the blower on a few minutes after being activated with Durametric. Previously it was blowing in less than 1 second. Without an ammeter I'll never know for 100% sure, but seems highly probable the old fan I replaced was pulling significantly more load than what is spec for this device.
Good learning experience for me troubleshooting electrical issues and also learned how to access the relays. If I ever have to do anything like this again it should only take a fraction of the time now that I've done this. The fan replacement was trivial but it was the troubleshooting that was of value in this particular exercise. Hope it can help someone else in the future.
Lots of good help on this one, so numerous +1s above. Solid guidance helped me nail this which is VERY MUCH appreciated. Just in time for my girl to get back into town this afternoon and for the weather here to jump to 90s+. I tip my hat to you gentlemen. Thank you very much.

Figured they would. Glad you got it sorted and it was that easy/cheap.

Problem was solved by replacing the cats. Now the post-cat sensors don't follow the pre-cat sensors at all. They stay very close to .7 V constantly on both banks, which means the cat is doing its job well.
After I got the cats off I could shake them and hear the honeycomb broken inside. I guess when I was tapping them with a rubber mallet when they were still attached on the car, I must not have been hitting them hard enough because I couldn't hear it then. At any rate, if I had to guess I would say this must have been from vibration/age. The good news though is that the insides of the cats looked good so there is certainly nothing wrong with the car upstream. So the car is 100% back in working order, running like a champ.

To finally close the loop on this debacle, the solution was to buy a new keyhead from Sunset ($125 + $6 shipping) and have my local dealer program the car with it (1/2 hr labor = $75).
If you read my troubles in this thread you will see this issue exhibited some strange behavior that made troubleshooting somewhat difficult to isolate: the issues were intermittent, a new battery fixed the problem for about 3 months but then it stopped working again, etc. My best guess about the original key remote is that it either stopped transmitting the correct frequency to the car 100% of the time as the hardware aged, or else it somehow became "out of sync" with the vehicle -- but I will never know about the later as the original owner did not provide me with the key codes when I bought the car so there was no way to try to "resync" using the code.
Either way, a new key head/remote did the trick for $200. Not bad. So in the future if you're having these problems then replace the battery first, and if the problems persist then just buy a new key head (996 637 244 42).

To be thorough and follow-up (hopefully for good), let me offer the following. Hopefully this helps anyone with the same problem in the future. Since the people on this site (JFP in PA, etc) were so nice to offer their time to help me, I think it's the least I can do.....I really, really appreciate everyone that helped me through this issue.
Everything I said above is true. Replacing the voltage regulator did get the voltage to stop bouncing around and I stopped getting PSM/ABS failures, etc. It made the car drivable again. However, on short distance trips to run errands I noticed the battery was slowly draining. I was able to keep the car operable by hooking my car up to my C-Tek 7200 charger/maintainer at night. In the process of all of this I had the wiring harness (the one that goes from alternator -> starter -> junction block) replaced because there was some resistance as you traced its path from the alternator. I believe there is a TSB on that and I understand that the issue is so common that most dealers keep the part in stock. However the battery was still being drained after all of that. I was so confused since I had the alternator tested at Auto Zone and their diagnostic said it was fine. Indeed, electrical problems can be difficult to track down.
It turns out my alternator was bad and I think it was subjecting the voltage regulator to high stress and caused it to fail. This is the chicken-and-the-egg problems so I don't know what caused what, but that's my best guess. As I would find out later, my alternator was working just fine at higher RPMs so highway driving was just fine. However, at idle RPM it wasn't working 100%. This explains why the short trips to run errands were problematic. I guess another moral of the story is don't always trust Auto Zone's alternator testing equipment. I'm sure it will tell you if your alternator is completely dead, but it certainly didn't catch mine which was half-working.
So I ordered a new alternator from Vertex. Part is 996-603-012-02 for my 2002 C4S manual transmission. Cost was $375 total shipped (after $200 deposit to return old core). Replacing the alternator is one of the easier things to do on this vehicle as long as you follow Loren's DIY and uncsrew the bolt just slightly and tap the bolt head with a deadblow hammer to loosen the back bushing on the alternator from the engine flange.