So here I am back doing what more people should do... updating with the outcome. Let me start with the short and sweet:
Thermostat and seems to be doing fine.
This was one of the items I originally thought would be the issue, but some of the other items are what made this all so much more confusing (and I'm sure I was reading too much into the various gauge positions as well). It wasn't totally stuck as suggested, – on the drive home the day I posted all of this, it was a rock-solid 11:00 at 72 degrees outside temp, with freeway traffic and open freeway. But, then I stopped at a grocery store, and the drive home was totally different.
First, that new radiator cap was a problem... didn't let anything out as it should have. The car was great, and really shot up to super hot so fast that I wondered if it the gauge was just screwed up all of a sudden. I killed it, coasted and let it cool down 100%. Topped off the system, and upon starting it, it wasn't pulling the water out of the reservoir. I popped the cap off topped it off there and we were back to normal operation... for about 8 minutes then the temp crossed the half mark and I killed it again. Not wanting to do damage, but not having a simple scenario for getting the car home (a freeway here was closed Monday evening) and doing it in small bits seemed to be the way to go.
After this one, I started losing water... fast. Anyway, I had two one gallon jugs, and would fill the system (which would leak with the car off and sitting still), fire up the car and drive it with the gauge where it should be. The moment I hit 12, I killed the motor, left the key on (letting the fan run) topped the system off and repeated this all the way home.
So the "cursed water plug" was quickly identified as the weakest link when the pressure built up, because it had a 3/4" split in it when I removed it (after taking the hint from steam coming from the back of the engine as the link on page 1 indicated).
New thermostat, new water plug (2 day wait from the local Mazda dealer) and I put a little over 1 gallon of 50/50 mix back in the system to get it on the road for a test. It was solid, seems to be leak free and dead on the 11 o'clock at freeway, stop light and semi-agressive driving. Looks like it was indeed the first suggestion... thermostat.
As an interesting side note, I backed the car out of the garage, into the street to look for leaks and such while the car was running. After spillage ran off of the underpinning and such it looked good... but something caught my eye. Right in the front of the radiator, was a big plastic bag (clear of course) spread out to cover just about 25% of the surface of my radiator on the passenger side. Not sure what role this played, how long it was there or anything like that. But it damn sure wasn't helping matters.
Anyway, thanks for the reassurance to keep it simple, and I'm just glad to be driving it back to work tomorrow. It's going to be 70* out.