My run was rejected because I had no clue you could only use 2 item slots, and since I am not able to disable the extra two I bought, I'm left screwed unable to participate in Kitty speedruns.

While you have 4 you can still speedrun, you just can't use the other 2. You can also delete the Game passes from your Roblox inventory, it gets rid of the slots. Or you can play on an alt account, there will not be a category for 4-item runs.