Before you start swapping engines, what happens if you physically lodge the choke flap open ?
As the weather is fairly mild it would not be used much anyway, so if it is the choke causing the problem you will know for sure if it's wedged open.
If you do it, use something large in case it get's pulled into the engine
When you mentioned the worn spring the mechanic was talking about the only one that is connected to the choke is the idle air corrector needle and if that fails it causes problems when warming up not while pulling away under load.
The spring reacts to the choke, it does not control it, that is done by the choke actuator (electronic)
I seriously think the choke flap actuator that is causing the problem but as you said it moves when the engine is turned on and off i suspect it is the temp sensor that sends the signal to the ECU which in turn controls the choke actuator.(still with me?)
Warm it up and wedge it open, it's got to be easier than changing engines

There is NO nucleus.