There are so many different problems in your post, it's difficult to follow.
I want it to have a master UI to select from, not have to copy/paste the HUD into each layout, as this causes issues if i update the HUD design, I want all layouts to get that same HUD change without me copy/pasting to every layout. Is this possible?
Yes, that's what Global layers are for.
construct.net/en/make-games/manuals/construct-3/project-primitives/layers
I am already using save game to slot and load game from slot with naming, then using goto layout to load the layout, but the players inventory is empty regardless.
System Save/Load actions save everything, so the inventory should be saved too. If not, this means you are probably clearing or re-creating the inventory on start of every layout.
Well, something else i found out today, my save game using the save slot, translated into the deployed build, so on a different android device, it loaded my last save slot, no inventory, just location, that i had saved in the development environment. Why would it do this?
This is not possible. You must have saved the location somewhere in the project. Run a remote preview, open it in a private Chrome window and check if you can reproduce the same problem.