The air bypass in the AFM isn't meant to be used as an idle adjustment... it adjusts the airflow across the whole scale. It's a bad idea to employ it for any type of fuel tuning. Get a real fuel controller or go stock... you're just going to melt something.
Just for my own edification.... Just how does opening something to atmosphere FIX a vacuum leak?
EVERY MKIII runs closed loop 90% of the time... '86.5, '87 and earlier are no exception.
You can't use Vf to tune. P E R I O D. It serves as a rough guide at best. And you can't even use it a a rough guide for open loop operation.
Heated O2 sensor? Sure, why not? Just use a relay triggered by the fuel pump relay and with it up for 12v. You can't just wire in a wideband O2 sensor though.... just narrowband.