>>537645660
Bumping my own question to share a semi-reliable solution: after some experimenting I found that the game always fills up the "next" Party Slot when you start rebuilding your ungrouped party, meaning if you link your companions one by one (the first to the MC, then the next to that one and so on) you can whip Larian's lol-so-random system in shape. Well, at least for a while. Technically speaking Party Grouping means all 3 companion enter the system at the very same time, and since the Party Order on the left doesn't seem to have any effect on this their place in the formation is randomized. Ungrouping then adding only one at the time basically circumvents this. While this method can deliver reliable results your party order is still not 100% safe due to the god-awful AI, which tend to rearrange it on a whim when you pass through narrow places or climb ledges. If anybody knows the ultimate solution to the core problem I'm still all ears.