11 C63. Trying to diagnose what situation I have here.

Eventually transmission started slipping and losing drive. This was on any gear.
It began making this metallic noise at the front of the transmission housing. Eventually, will not drive at all.

Logically it pointed to torque converter. We pulled trans, changed oil/filters/etc (for the 2nd time) put in a reman'd tc and started car.
All was good for about 2 minutes sitting at idle, then started to make a faint whining noise again. Did not drive, just shut it off.

Yes, we prefilled tc, clicked it in properly, etc. Job was done perfectly.

There were originally metallic fragments in the oil. So yes, it's likely the whole thing is gone. But I do not understand what is rotating at idle that is shearing itself. Any ideas?
It can't be conductor plate because that doesn't correlate with the noise.
I guess I'm fine putting in a new transmission in but there isn't exactly an abundance of available ones out there; especially ones from cars that haven't been absolutely pancaked.
Of course, having the car out of commission and the added expense is not ideal. I haven't really heard of transmissions failing on these unless they are heavily modified.

transmission oil pump? whining noise leads me to suspect the bushing/bearing

So many components in a transmission that can fail, I'd pull it and send it off to SHR for a full rebuild.