No. I buy in full pallet quantity several times, even going through their bid desk and it's always the same price.
I'd say half an inch
There is no way to determine a shelf life to opened bags. We would recommend using them ASAP. Concrete only has a 1 year shelf when sealed. After that the material will not work.
As a fencing contractor I usually only use an 8" hole for 4x4 posts, and at 24" deep, I use 2 bags average. For a 12" hole with a 4x6 post I had to use 5 bags, so you'll likely need 6-7 bags per hole for a 4x4 post in a 12" hole.
Yes, it will corrode the aluminum. That will not be a good idea.
yes using on pressure treated posts
No, you will need to fill the hole 1/3 full of water and then pour in the dry fence mix until it is full.
There is .375 cubic feet of concrete per 50 lb bag. Concrete weights around 146 lbs per cubic feet when dry and cured. So you are looking at approximately 55 lbs plus or minus.
64 bags per pallet.