Apart from the explicit question about stabling, I think everything else has already been discussed in this or other threads.
The 442s will not be retained for use on residual London to Brighton services. They might be retained for a relatively short period for Eastbourne peak extras, but only until other 377s become available following 700 introduction.
The suggestion of 7 x 319s replacing 7 x 321s has been all but denied in the '321 to Scotland' thread, the reasoning being that introducing a micro fleet of 319s onto the GN for a very short period would not be practical. I don't believe the LM 321s are in the picture for Scotrail, which was the other suggestion. LM wouldn't want a micro fleet of 319s either, IMHO.
The Bletchley depot building isn't used by LM any longer, which makes it a convenient location to temporarily base the new units for the few weeks needed while undertaking high speed AC testing on the WCML.