Happy Ending
No reloads, only factory fresh ammo.
After the extractor was free of the copper buildup, I re-assembled and took it out to the range. I took my Springfield XD with me as well as a control against bad ammo.
I first shot 10 rounds through the glock then 10 rounds through the xd of Independence fmj (Anyone know who makes this? It it good stuff!). I then repeated for Remington UMC FMJ. I alternated this through 50 rounds of each, with the glock getting the last 10 of each box.
I shot deliberately, ensuring a very firm grip to prevent limp wristing, keeping my shot placement simply COM at 5 yards since I was concerned only with function, not accuracy.
I had no failures of any kind in either gun. Each brand firmly cyled the slide of each gun.
Next, I went to the low end Walmart ammo I had been consistently shooting recently. Starting with 10 rounds from the glock, I immediately noticed significantly less force from the round. The glock cycled, but not nearly with the robustness of the other two brands. I also noticed that it was ejecting a soot onto my left hand as well, which the other two did not do. After noticing this, I finished the magazine and then inspected the chamber. Sure enough, the extractor was beginning to foul with this ammo, but had not with the others.
I finished off the box with the Springfield, which had no problems with it.
My final conclusion is this. My glock does not function well with the bottom end walmart ammo. Since that is primarily what was shot through it before the problems began, and based on my observations today, I concluded that the excessive fouling I witnessed on the extractor was caused by this ammo and insufficient cleaning of the extractor. Over time, the fouling built to the point that it prevented the extractor from functioning. The lower perceived power of the round may also play a part, but I can only speculate on that.
I will simply not shoot the cheap stuff in the glock. A few bucks savings on a box of ammo is not worth it if the ammo damages a $650 gun. Anyway, case closed and I am happy to have my glock functioning again.