Do you like anticlimaxes? I sure do!
We tried the cheater bar again at the FLBS, with no more success than previously:
The answer was to remove the right crank and beltring (without detensioning the belt - there was no way to do so anyway). This allowed for a firm grip on the assembly from the right side. Then using the PVC pipe as a pusher, the FLBS used the world’s biggest rubber mallet and the fury of a thousand suns to just hammer the BB out. Took like a dozen blows.
(if anyone’s shopping for the PVC pusher, a DN25/G1" connector works wonderfully)
Upon inspection there is nothing wrong with the bottom bracket or the shell. There is no glue residue. The replacement aluminium bottom bracket just popped right in without a pusher also, so I guess it was just arbitrary variation in the plastic casting.