2019 Mercury (Tohatsu) 30HP EFI 4 stroke kicks over but stalls immediately

[Jethronone]

I recently replaced the fuel pressure regulator on this outboard as it was overflowing fuel out of the VST.
This solved the overflow problem but I still had sluggish acceleration under load until I emptied the low pressure fuel filter and cleaned the injectors.
Bingo the boat went perfectly for about half an hour then stalled and wouldn't start again for about 15 minutes.
Got it back to the boat ramp and to home where I could get it to start but would only run for about 10 seconds before stalling.
While it was idling I checked the fuel pressure at the fuel rail and it was exactly as per the Mercury manual specifications between 42-43 PSI.
Today I removed and checked Idle Air Controller which did look a bit crusty but the resistance was as per the Mercury Manual at 27 ohms.
Checked the connections on the IAC and the Mat sensor and seemed OK.
Now I put it all back together and it won't even start properly. It kicks over and fires but dies immediately.
I checked and cleaned the injectors (but only with carb cleaner) but they look ok and it didn't help my issue.
I am thinking maybe the IAC should be replaced but wondering if anybody has any other ideas as to what I can do?

 

[Jethronone]

Update if anyone can help. Today I tried starting the motor with the throttle open to allow air into the throttle body which I gather is what the IAC would do. Yes the motor starts and runs rough then conks out after about 20 seconds.
If I then try to start it at neutral idle it just coughs then stalls.
I am thinking this reinforces that the issue is probably the IAC but appreciate any other thoughts or tests I could do. Is this where I should start or is it more likely Throttle position sensor or MAP sensor or is that just asking how long is a piece of string?