That sucks.

I guess it could be many different things. Here's a scenario that fits the symptom, albeit rather unpleasant.

The new starter was jammed into the flywheel which is why it would not disengage. However the jam is due to radial preload against the flex plate. This radial load has caused significant damage to the rear main bearing and journal, resulting in the squeal and enough interal friction to require throttle to keep her running.

Not pleasant but it fits the symptoms.

I always assume the worst; seems to find me often

Good luck, Kitz