It just moves the open can to the upper-most available free slot in your inventory.

This is a feature, you can be hungry and or thirsty, to provide a more gripping incentive to find food and water.

Confirmed. I had 3 sets of purification tablets in my inventory, one at 100% another at 75% and another at 10%. Dragging the 100% to a water bottle used the whole thing up. Same with the 10% tablets.

Same, I've found notes on the floor and in vicninity, right clicked them and chose "read note" and hit the use key and nothing happens. I've put them into my hands and hit the same key and nothing has happened?

I've experienced this also with nearly all kinds of items. If the item is in my vicinity and I drag it over to my character, which is full already, then I take the item back to "vicinity" and let go, it vanishes. Confirmed several times.

Can confirm, I found a pristine fresh orange and pristine fresh kiwi on floor at 0% and could not eat them. I assume this is meant to be their "skin"? Maybe a new model or title could help avoid confusion.