just tested it, you could only edit existing item slots, so there must be another address that tells the game how many total slots you could useusing your values as a guide looking at memory but this game completely changes.
for example at the start of a new game the addresses are
[Vital Star]
04100000 a0802298 00630a02
[Devil Star]
04100000 a080229c 00630b02
have you had any luck getting all items/infinite where it doesnt mess up.
i tried from a new game and you dont get the item unless you buy it, tho memory doesnt change, also if say slot 1 and 2 are items, next item key, you cant use an item in its place unless you have either bought it or collected it.
later in the game i found all my items get messed up, so probably all weapons/items ect not possible
and what do you mean by all your items got messed up later in the game?