Sounds like you are steadily getting somewhere.
I wouldn't run additional advance on these motors, I know it's been advised but don't understand the logic used to decide that (any thorough explanation would help illuminate if anyone has a confirmed view).
What I get if going over 6degree base setting is that the base idle increases and brings the floor setting too close to the ecu control logic, as you are getting.
It seems like this you have to run the throttle plate too closed to get ideal tickover (confirmed by your observations of IACV fully closed) down to factory settings.
If the throttle plate is too closed, then each step of IACV is proportionally greater (of the flow through area) than if it where more open from base idle setting. That could be giving you the error response from ecu as expected step request results in too greater response.
I'd set the timing at maximum 6 degree (on older more worn cam chain you can see a flutter as it flaps a bit) so set it that you get no more than 6 degrees with a strobe. Then check base idle.
Difficult starting, that compression psi is not that high if it's generally at 120 average, I would expect more cranking to get it going with that figure. General feeling for me is that you need to be 140 and above to be competent at most things.
It's certainly not a disaster (plenty of engines run for years at that level) but needs to be taken into account for your assessment regarding starting.