After removing all 3 ABS fuses from the fuse box (the 10A, 20A, & 30A) last night and reconnecting the battery, I decided to turn the ignition to "on" under battery power (without cranking the engine). Cracking open the bleeder valve for the troublesome wheel (driver's side rear) and trying to pump still produced no fluid through the line...so I closed the valve and pumped the brake pedal about 20 times total with swift, forceful pumps. I then removed the keys from the ignition, cracked open the bleeder valve again, and tried pumping. This produced some fluid through the line, but not as much as with other wheels that are functional. I decided to turn the ignition to "on" under battery power again, pumped the pedal with the same cadence and pressure as before, and then switched it to off.
This seem to have fixed the issue with fluid flow from the driver's rear wheel.
Cracking open the bleeder valve and pumping now saw fluid flow normally through the line. I must note however that there wasn't much air in the system at all. The pedal still felt firm while pumping. Just to verify that any possible air didn't travel to any other wheels upstream (if it was caught somewhere in the ABS module), I went ahead and bled the other wheels. Same thing, no air, perfect fluid flow, great pedal feel.
However, a new problem seems to have come up...
In the cross circuit wheel from the driver's rear - that is, the passenger front wheel - even though fluid flows normally through the line during pumping, there is a noticeable hissing/squelching sound that seems to come from somewhere near the brake booster/master cylinder/ABS module area. The pedal doesn't feel as tight as the other circuit's wheels (driver's front and passenger rear), and it doesn't feel as tight as the once-troublesome driver's rear wheel. I've pumped the passenger front wheel about 30-40 times in the same method as the others, and can confirm that there aren't any air bubbles passing out through the system.
Here's a video of what it sounds like: PassengerFrontWheel_BrakeBleed_AirSound - Streamable
Sounds like it might be a master cylinder issue? The brake booster itself is able to build pressure nicely when I pump the pedal, and can hold it indefinitely (several weeks+).