Sadly, I think your number 1 assumption re manning numbers is flawed. No matter what is "promised" now - all will be up for negotiations when coalition talks start.
If by cutting another 20,000 service people the politicians can spend more on the NHS - that is the choice they will undoubtedly make.
I'd suggest all sacred cows will be rounded up for slaughter in the next review (Reds? Ceremonial Guards? London bases? etc etc)