Heyo, I can't answer all your questions but I can give you some information.
Resonation is a resonance frequency (I'm sure you knew) and the frequency of a sonic wave has a length. So if you put in 23" GPs and that eliminated the drone, then I would leave it just like that. If you shorten it then you could miss the resonation and it will do nothing for the drone. There is a calculation you can do to find out how long your stuff should be, it's pretty complicated, but I think most people do with 18" pipes.
I don't know what to tell you about the other side though... that flat spot is pretty annoying. You can probably replace the whole length where it goes flat then just put the GP's in there.
Check out what someone posted here about a counter frequency pipe to eliminate drone: Fancy Tech: How to Get Rid of Exhaust Drone, works great! - Nissan 370Z Forum
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