About levels, I don't really see why it's a problem that you can't reach the same levels at the end of the game. In every games, only players doing every side quests and combats ande stuff that gives XP reach the higher level. I don't feel sad because I don't reach it. Level 10 is not the cap anymore so everyone will be able to reach it.
I don't agree with this, because the highest level possible you can achieve will be calculated with the least party members in a group in the eventuality the XP would be shared among characters. So if the devs have decided for example the max level is 13, because they haven't implemented any spells or abilities above this cap, and someone wants to play the game solo. In the case it's the proposed shared XP calculation, the 6 party members will have to share the same amount of XP one character can get to reach 13. And honestly, I am not sure they would get to level 10 at all.
Because if some players here want their party to stick to the max available, there are others who want to play solo. And in my opinion, both should be considered valid in a game like this.
That's why I think it's a much bigger stretch to go from 1 to 6 party members than from 1 to 4 in this scenario.
But all things considered, I think the shared XP is impossible cause how would it work with 15 companions that you can switch according to your playstyle?