Could the Backpack use the "Back" slot (the Curious Shulker Box uses that one) if it recognizes that mod is installed as well, instead of the "Backpacked" slot?

You can change it by overriding it with datapack. The file you need to override is data/curios/tags/items/backpacked.json and change the slot definition inside of it to the one from Curios Shulker Boxes.