If you're upgrading to level 5 chances are you're doing adventures. In which case a week without troops is nothing. It can easily take that long just to replenish settlers. Planning for some adventures takes more than a week so where's the problem with not being able you train? And the buffs. If your buff buddies are really 'buddies' then they should understand that you'll have a week with fewer buffs (plan it right you'll have buffs) because as you said, the community spirit part of the game means they won't be able to produce buffs for you when they go to upgrade their provision house.
Yet again this shows how people react when given something called a challenge. The whole point is to make you use your brain to plan out how you're going to upgrade these buildings. I havent yet upgraded mine to 4 or 5. Now I know this I'm sorry, but I'm rather looking forward to the challenge of being without a provisions house for a week. Gives my brain something to work on in the game