The first question is answered in the +3 CP/M manual (search for 'Memory Map'): 4-5-6-3 for CP/M bank 0, 0-1-2-3 for CP/M bank 1, and 4-7-6-3 for CP/M bank 2. The arrangement (doubtless by design) is very similar to that used for PCW CP/M, where CP/M banks 2,3,4 etc. are the same as bank 0, except for the 4000-7FFF range.
The design of CP/M is that programs ...
I can't add much to the above answer, except that the additional modes would have been there either because they were a harmless side effect of the primary 64k CP/M mode that would have cost more to mask out than to leave, or they were there to mirror modes in other Amstrad Z80 boxes and help porting.
Richard Altwasser may know for sure, but he's run away.