If you have multiples of the same mag, it will only load the one with the highest amount of ammo. It will not cycle through the mags, this is a feature of the arma engine. With multiple canteens, you have to select each one into your hands to refill them.

Swap the mags in your inventory. The active one will be up and/or left of the others. Known 'feature' of the arma engine.

Are you talking now about the physical action of dragging another mag that has a lower value of bullets in it to the gun in the inventory screen?

The mag acted on when you press reload will be the mag that is top left of any other mags in your inventory. Simply move the mag you want to reload so that is is in an inventory slot up and to the left of any others.

If your use your scroll wheel before hitting F it will give you other opions of things you can do... like choosing a different Mag to use when you reload. I bound my mouse scroll up and down to my keyboard since my scroll wheel doesn't work. I've never had an issue trying to reload mags. As far as the canteens go, what are you looking for a 1 button press to auto refill all your canteens? This game is going for some level of realism, hence having to put each canteen / water-bottle in your hand to refill.

To note, the mouse scroll does not show every mag I have. THat is the issue I am trying to bring up.