Fiddling around with the inventory is pretty fun at current, though I suggest a couple things for consistency and usability sake;
Keep B reserved as the universal 'exit menu' button, like Esc on keyboard. So you don't have to press the same inventory button to exit, which is more convenient if 1 is the default.
The split stack functionality can just be handled by Z instead.
The possible place one item at a time functionality could also be handled by Z when an item stack is selected.
The possible shift click functionality could be handled by C
Fiddling around with the inventory is pretty fun at current, though I suggest a couple things for consistency and usability sake;