Why aren't they running Richmond/Ealing trains to Edgware Road (using
slots and trains from the none-existant circle line)?

The platforms North of High St aren't long enough for D stock trains.

I think he meant using 6-car C-Stock. These certainly have clearance
issues in the Dagenham area (meaning no through routing Hammersmith
Met - Upminster), and may have on the Richmond/Ealing branches. There
also may not be enough C Stock.