I certainly would agree with the 3 COM muxes into 2 scenario if DVB-T2 is implemented.
Here's a scenario:
Perhaps one of the temporary muxes could be a pre-700MHz clearance version of a post-700 MHz clearance mux?
Like Mux B and the "Advance Network" HD multiplexes before switchover.
"Mux 8" could become the second of two COM muxes after switchover, with the first COM mux incorporating SD services on SDN and Arqiva multiplexes. Not all SD channels currently carried on the multiplexes would migrate: any channel duplicated in HD on the HD COM mux would go, and some of the other COM channels would go as well, including ITV3, ITV4 and CITV (see below).
On the PSB muxes, if they went DVB-T2, one of the three PSB muxes could take over the planned "Mux 7", giving you BBC-A, D3&4 and PSB3 incorporating Mux 7. The current Freeview HD mux would close, with content moving to BBC-A in DVB-T2 or D3&4 in DVB-T2. One of the planned BBC services on "Mux 7" could also move to BBC-A, leaving slots on PSB3/Mux 7 for ITV to accommodate services pushed off the COM mux.
You could (hypothetically) then have BBC One HD, BBC Two HD, CBBC/BBC Three HD, CBeebies/BBC Four HD and BBC News HD on BBC-A
ITV HD, C4 HD, C5 HD, Film 4 HD, More 4 HD on D3&4.
ITV2, ITV3, ITV4, CITV, BBC Parliamant, BBC Alba/S4C in SD plus E4 HD, BBC RB301 on PSB3 incorp Mux 7.
The COM muxes could be
20-22x SD channels on new COM1 (DVB-T2 MPEG4)
5x HD channels on new COM2 (QVC HD anyone?)