What I am doing on the 4x board (the one compatible with the existing uBITX) design point is:
#1) it fits in the space I cleared on the uBITX board picture,
#2) it selects 3 of the relay sets with the TxA, TXB, and TxC uBITX drivers for 80/60m, 40/30m, 20/17/(15)m,
#3) if any one of those 3 drivers is picked it selects the TXD driver (on the 4x board) which is driven by a 3 input diode dot-OR from the TXA,TXB, TXC inputs. The TxD driver has the 4th 10/12m filter on the NC contacts so you don't have to have 1 LPF relay energized at all times and may not require K3 if it's the highest band LPF, like on the present uBITX. .....haven't decided yet).
#4 yes, disabling audio M1/M2 with the same RF relay is strange.
73 Kees K5BCQ