Logic only works if you have background info and experience to understand how the mechanisms work.
No, IACV only makes minor +/- adjustments to idle-speed. It doesn't control ALL air going into engine, just bypasses minor amount of extra air around throttle-plate. Primarily to increase idle-speed on cold-start when throttle is closed. Above 70F, it's not used at all. You can remove it completely and would only have slightly rough-running on cold-starts (might have to hold throttle slightly open), depending upon ambient air-temperatures. But it would still start just fine.
Your issue is you're not getting
ANY petrol into engine at all. As confirmed by it running fine when external source of fuel is supplied. You are just 5-minutes away from fixing this.
A lot of old-mechanic's "fixes" come from carburetor days. Doesn't really work for EFI. Engines, intake and fuel-systems are sealed and there's very little evaporation that causes varnish like in carb bikes. Most non-start issues on EFI bikes are purely due to electronics: sensors out of spec or adjustment, broken wiring (mice just
love that soy wire-insulation), or some other electrical fault. Luckily those are binary in nature and easy to verify (Is that darn plug connected or not? Is there or isn't there power? Does test-light turn ON or NOT? simple).
It's just like troubleshooting why your kitchen light doesn't turn on. Sure easy impulse is to replace bulb. But what if that didn't work, now what? You've now got spare bulb and injector! Well... is there power in bulb-socket? Then if there's no power, what controls sending power to and removing power from that bulb socket? Simple logic if you understand mechanism behind electricity.
I've had racecars sit for 3-5 years waiting for rebuilds after crashing. This one's had 4 engine replacements (got too greedy with that boost-knob...). Just disconnect wiring, remove fuel-rail with injectors, remove engine, put new one in, re-connect fuel-system & wiring. Once fixed, turn key, push button and VRROOOMM!!!! Of course, 1st couple times, I was too rough with wiring and broke some, leading to hours of testing and head-scratching to find which one I broke. Never had issue with petrol or fuel-system though.
BTW, here's REALLY common problem with injector connectors on older vehicles. There
are ways to determine health of wiring without destructive analysis...