I added a few checks inside the Shop/Inventory code and re-uploaded it to stop the player being able to open the inventory from inside obj_player. The inventory system is also a 2D array that can stack items, while the Battle System does not have stacking. Checking for player input also needs to be synchronised (like HELD_LEFT / HELD_RIGHT is not present in the battle system - it's only keyboard checks).
It's definitely doable but it also depends on your coding ability and ability to understand my code, which can be hard if my style is very different from yours.
These problems arose because the files are from two different tutorial videos I did and I was definitely not thinking about integrating the two when I designed them.
It makes sense for me to go back and change some things in both of them so that they are easily mixed together so I think I'll be doing that. It feels wrong to change the classic battle system to stacked items but I think it's for the best!