Thanks, that is super helpful information! Somehow I didn't come across your older post about the EVAP system during my searches. I had eventually figured out that the line to the rear bumper goes to the carbon canister/valve housing (# 8 in this diagram), but haven't had a chance to do more testing yet.
It took almost 500 miles, but it did eventually throw a code for incorrect EVAP pressure with that line capped. That is helpful to understand that it is an air intake line rather than a vent.
I guess my next step is to test the shut off valve, tank venting valve, and vacuum limiting valve for proper operation (all the ones near the carbon canister).
I only came across one other post of anyone mentioning smoke coming from the rear bumper during a smoke test, and it was immediately dismissed as caused by someone not knowing what they are doing, so I'm still curious if my smoke testing method is correct. Do any of the other valves need to be activated during the test other than the purge valve below the intake? I'm doing all of this with the key off, so nothing else is powered. I still haven't found any other leaks during smoke testing.
Anyone know if any of these symptoms point to failure of a specific component? One of the numerous valves? The carbon canister itself? I need to get this thing through smog testing very soon.