Yeah that is GM troubleshooting via error codes for ya. lol
It really sounds like the tray that ejects the cassette may be jammed, my guess is still that it warped a bit from being left in there so long.
Years back I had something similar happen, it was from a regular cassette being left in there too long, I have also seen CD'* warp and have trouble ejecting, of course the deck has to get really hot for it to happen, it is a possibility.
And if it is warped bad enough you are going to have to remove the deck, open the unit up, and help eject it from the rear part of the cassette, it may even need some power going to it while doing so, but you will have to be really careful working with anything getting juice, you don't want to get electrocuted.
You should be able to roll the gear that lifts the tray and ejects it by hand, that would be the safer route, if the tray is already lifted the cassette up then that won't be necessary, a shove from the backside should be enough.
If all else fails I'd be happy to help you find an aftermarket deck, unless you'd rather check a salvage yard for a stock radio like you have.
2006 Pontiac Grand Prix GT with SC
1996 Buick Park Ave NA