So in theory, my character could spend all his hit dice during the "short rest" period of the long rest, fall short of going back to full, and then in the morning take another immediate short rest and spend the dice I just got to top me off?
This is not a necessarily a critique. I have been playing with a version of what you suggest (not the spell slot stuff, just hps and hd) and this has kept me from implementing it - so I am trying to think it through.
I think what I came up with so far is, Short rests are cut to 30 minutes but otherwise work as normal. Long rests return half your max hps (so you could end up at max if you are only lightly hurt) and you can spend HD to get more but you can't regain more than half your level minus any you used this way.