I just opened the game after a few days of a break and the stash thing... needles to say I am not going to play anymore until it gets fixed.
I do have a possible quick solution that can maybe be really implemented within the code structure.
As I understand current shared stash is triggered by the fact that Companion leaves your party and from the code lvl it can not really distinguish if its a permanent farewell or just old good stay at the camp.
So as a proposition for a quick easy fix (while you can work on more complicated solutions for the rest of inventory issues). Instead of forcing us to have Companion items, the moment Companion leaves just force opene the inventory with a question "would you like to take any items before your Companion leaves the party?" Or something in that style.
I know it's not a prefect fix and all other issues still remain valid but at the very least it will make the game playable again
With best wishes Larian I still love what you've made!