What about spark? Have you checked? Look at the carb base gasket make sure it is the correct one. And make sure it is on correctly, all holes must line up.
Get rid of the electric fuel pump. If you have the Vro put it back on and just delete the oil side. If not buy the pump off of an 88 hp and put that on. What happens if you pump the primer ball?
That motor has the tell tale connected directly to the thermostat. pretty specific to the 2.5 merc motors also done on the smaller 25 hp motors. I think you are looking for problems that are not there.
I do not know how you are getting a 2026 delivery on an NLA piston. A little more looking on your part. I can't put the website up because it is a competitor. they say 10+ in stock have found them to be quite on top of their inventory.