Sadly, no good news.

I had already lowered latency to 1ms before posting this thread, so there's that. I did change the USB buffer size to 1024 as in your screenshot just for kicks but that didn't change anything either.

So I tried my FTDI friend, or rather, THIS one from my local electronics store. It was flaky almost immediately. I only managed to complete a read once, after which it said the same "invalid checksum" error. All other attempts it would fail at some random point and force me to yank the ECM fuse to get everything back. I thought it might be bad connection between my test leads and the socket, so I desoldered the socket, soldered together the TX and RX pins, then soldered wires directly to the GND and TX/RX pins. This changed nothing, it was just as flaky as it always had been.

FT_PROG reports that the chipset on both my flaky device and my perfectly-working-except-for-EEHack-Reads ALDLcable is the FT232R. Neither is FT232B. The cable seems to have zero issue talking to the car otherwise, it just won't let me read the ROM in EEHack. My little dongle though, nothing doing.

I suppose I could drop by there and pick up the Adafruit version just to see if that would do anything different. If it doesn't I can always return it. I'll do that tomorrow after work. But for now, I figured I'd drop that update.