Alright, so I desoldered the CH340G chip and replaced it with a new one. It works now. I pray to God, PLEASE to not give my circuit any more issues... This is REALLY weird since I have no idea when and how the chip got damaged. When it stopped working again after my little crystal switcheroo "solved" the problem, I ran a loopback test on the CH340G by isolating it from the 644P by cutting off all RX/TX connections, then wiring the RX and TX together. The test didn't work, which led me to think the CH340G was probably fried. I replaced it with a new one, and it's now working again. For now, at least... 