OK, so somebody reserves all buildpoints. Then the other team destroys 2 turrets/acid tubes and those 16 buildpoints become available to anyone? Is that really a good idea?
No, they won't become available to anyone with what I said

Let's see, when someone reserves 20BP, it means that until the reservation is lifted, non designated builders cannot build anything, unless there is at least 28BP available ( for turrets, medi, DC ) or 30BP available ( for the rest ). This lasts throughout the whole game, no matter what happens until the number of BP reserved is changed. Ie, until you detect a problem ( no designated builder left in the game ) or until a designated builder selects a new value for the "reserved BP" variable.
The problem is when all designated builders are either dead and can't spawn or have guns and can't switch to ckits. Should the reservation be cancelled then? If not all, how many build points should be made available? And what kind of "the rest of the DB stuff" are you refering to?
The rest of the DB stuff refers to the buildings locked by a designated builder. I suppose that when you decide it is time to unlock all buildings, then you should also reset the "reserved BP" variable to 0, nothing more, nothing less.