That plastic screwpiece that attaches the sprayarm

I need to get the spray arm off to inspect & clean underneath it.
I got this plastic piece to turn a bit, and it’s partially out, so the threads are a little exposed, and the spray arm is wobblier. I can turn the piece by hand. But the piece won’t come out, and the arm won’t come off. I can’t get it to screw back in again, either - it just spins.
How badly have I messed it up?
Is there a way to get it off?
I’d be fine with destroying the piece, if I knew I could replace it and if I wouldn’t damage anything else.

What I called “sprayarm” is actually the “Wash Arm Assembly” 8268332.
What I called “plastic screwpiece” is actually the “Lower Spray Arm Hub” 8268342.
The photo shows how far I got.
What’s next?
hubsofar.jpg (58.0 KB)