You seem to have covered everything., well almost.
All these engines have did this with me over the years , so its not only the MCI.
I just sold a 1998 cart that was doing this exact thing but the person didn't care. It had the Pre-MCI engine in it. You could drive it for 30 minutes. You would stop and let it sit for a minute and hit the gas hard to take off and it would spit-spatter
for a few seconds and then go. Ran great except for that one little hiccup
I also ran into this during the 16 years I worked at a golf course that had both MCI and Pre-MCI engines. I was around when all the upgrades took place trying to solve all these issues they had. The intake valves were made soft on all these engines and after time they would pull into the head, especially if someone was running it faster than it is supposed to go.
The only difference in the spark plug you are using and the ones I used is it takes a 5/8th socket to remove them. I normally tossed them because of the 60 carts we had I didn't want to be changing sockets. I used the BP4ES in the PreMCI and BP5ES in the MCI's.
Have you tried a new OEM carburetor from ezgo ? They are high priced and most people don't want to pay so they try and clean theirs. There is a part under the main jet that can't be cleaned without removing and I never had any luck getting one out. Ultrasonic Cleaners are supposed to work but I just never bought one. I just pay the price for a new carb and normally that gets the problem gone
From what I was told in 2008 that the Robbins/ Subaru could not meet federal emissions so they opted out and went with the Kawasaki . The MCI engines were made starting in 2004 until 2008 when they starting putting the jerking, jumping loud Kawasaki's in but that another story