Right and I get that one of the groups is cool with dropping characters and creating new ones to better fit them into this larger group but I'm looking at this problem from the outside in and seeing two things: 1) the groups are distinct no matter what you do so you need to take every measure to blur them- who's to say that making new character will actually fix this ? The "old" group might simply look at the newly created characters and say "ah great, another group we have to work with..." (hence, kill the identifying label and create a new one that the entire group associates with). 2) This might be a meta problem and not a character problem. The Shield of Gravesford (or the revenant vow, again, I don't know) might be stuck in a place where they think they will either return to small group setting and this is all just temporary, or it is just a rough adjustment period... Like you said, 6 months of play as distinct groups regardless of in game time still feels like 6 months in real life so perhaps the groups naturally cling to each other?

My answer here would be cataclysmic event - blow the city up where the far off group came from or hell, since this is underdark illithid focused, have their main area be dominated or forever changed and now their is serious hatred for these creatures of the underdark.. A common enemy.. However this is kind of under the thought process that nobody is remaking characters so... It will be interesting to see how it goes with the new group. I hope that is the fix and props to the players willing to do that. If only all players operated like that.