I'm actually going with the first group. According to Nomura, Xemnas is the strongest member, Roxas is potentially second, while Xaldin and Lexaeus follow.
Assuming Roxas and Xion's power combined mimics that of KH2 Sora after he combined with Roxas, it can be said that Xemnas can take out Roxas and Xion. Xaldin is the next strongest in Group 1, while Axel or Marluxia are next in Group 2. I'll go with Marluxia, and Nomura dictates that Xaldin will win that battle. Same can be said with Lexaeus vs. Axel, with Lexaeus being the winner. Even if Nomura never confirmed Lexaeus' strength, Axel did recognize Lexaeus' intense power in CoM when speaking with Zexion.
This leaves Xigbar, Vexen, Zexion, and Saix remaining. Aside from Xigbar, I believe Vexen and Zexion are pretty weak. However, since you said we can include KH2FM battles in this, Vexen's power to create an Anti-form of his opponent can be extremely useful. Days even showed us an Anti-Saix, which proved to be at least similar power.
Group 2 has Demyx, Luxord, and Larxene remaining. Demyx seems to be the weakest member canon, but is one of the hardest in KH2. However, he is one of the easiest Data battles. It's hard to confirm Luxord's power, but something about the KH2FM cutscenes makes me want to believe he's powerful. Larxene may not be one of the strongest members, but her battles are pretty vicious.
Here on it's a 4 vs. 3, giving Group 1 the advantage. I'm just going to go ahead and say Saix can take down Demyx, and I don't think anyone will argue with that. Luxord and Larxene are indeed tough ones, while Vexen and Zexion just don't seem all too strong. Xigbar can probably take out Luxord. I'm basing that on Xigbar is definately a tough one given his role in the KH series, while Luxord is "probably" strong. Allowing the KH2 battles to come into account, a lot more people had trouble with Xigbar while many people found it hard to lose to Luxord. Even though Luxord's Data battle is what his real battle should have been like, Xigbar's data still proves much tougher. This leaves Vexen and Zexion, probably the two weakest CoM members, vs. Larxene. Since we're counting KH2FM battle power in this, we can throw in Anti Larxene, Vexen, and Zexion vs. Larxene. If it's still possible Larxene can come out victorious, which I don't think is possible given Anti Larxene, while probably not as powerful, is a strong addition to Group 1, we can say whatever energy the remaining Group 1 members have can probably take her out.
However I realize this can have an error due to it not being Final Xemnas. However I still believe Xemnas can take them both out, since isn't Xion doing nothing but draining Roxas' power anyway? Xemnas can take out a full powered Roxas, but since Xion exists, we only have two "half pints". Xemnas can already take out the whole, and splitting it into two lower forms wouldn't be much more of an issue. Some argue that it might, however it seems Xemnas' power was far enough ahead of a fully fleged Roxas to be able to take out two weaker forms.
I know battles can't be mapped out, but I'm just using what Nomura stated and the Data Battles to back me up.
Just my two cents.