Hi
Not sure i understand all your questions but will try some answers...
The tooltip about "170 ammo uses 60 per battle" tells you that that stacks has stored (on their backpacks and wagons if they carry some) 170 ammo and will use 60 every turn they actually get involved on a battle. Ammo is not spend if you don't fight.
So you can fight on the wilderness (no ammo resupply) for two turns at 100 efficiency and on the third turn some units will have an out of supply penalty (60x3=180 and you only have 170).
The same applies for supplies. The tooltip tells you how much they currently have and how much they eat up per turn.
The stack will always try to replenish in the region they start the turn if that region has some supply/ammo available.
The regions supply levels (see the tooltip for a region) is given in "Chits" so is a different measure that the points used for units and stacks.
Depending of the size of the structure, this chits will represent just supply or supply+ammo. See the manual for details
This chits are produced every turn, but for what i know don't accumulate over turns. So a region with a fort with a supply level 5 will always have 5 supply chits stored, no matter if you have units there using them or not.
The supply level of a region can be reduced temporarily by pillaging, and i think also for winter weather.
Of course, destroying a structure like a for or a depot will eliminate all the supply they generate on a region.
A very useful hint (for me at least) is that as a rule of thumb, each of this chits equals 5 supply points. So a region with supply level: 5 will produce enough supplies to feed a stack that eats up 25 supply points per turn (as said on the tool tips we talked about above).
About the ship question: not sure

I think yes, it will pass the supply if the units need it.
About wagons: they don't use any supply and carry a lot that they share it with the other units in the stack.
And certainly they replenish on a structure (as every unit does) up to their maximum supply/ammo allowance.
Check this for more details
http://www.ageod-forum.com/showthread.php?t=10790
http://www.ageod-forum.com/showthread.php?t=10128
Hope it helps!
Regards