I think it could be any of the above things you mention. Frankly, until recently I never noticed Cherry Barbs having any unique or interesting behaviors (isn't that unfair?), along with Danios, they were the first fish I ever purchased. Eight months later and all my Cherry Barbs are still alive - while, believe it or not - only one of my Danio's is because he KILLED by chasing/stressing to death, his other school mates.
I never noticed any sexual behavior and have tried and failed to sex my cherry barbs other than looking for rounder plumper fish as the females - and what is rounder or plumper or drabber or brighter is rather arbitrary (all mine look fairly bright), so you may have found a new male in your barb group. I have seen close friendliness in the same sexes of other groups - mostly males actually, because they just seem to like hanging around with members of the same species and aren't fish that show dominance - but then I always tend to mis-ID my fish as males so it's hard to tell.
The whole thing sounds really sweet to watch. I recently started building a big cave out of about 40lbs of slate rock I bought (which was crazy expensive). I built them for my shy Pleco's and they do use it, but the Cherry Barbs use it even more - to the point where I worry about them not getting out of there in time to get any food. One definitely looks VERY pregnant and had a CoryCat following her from spot to spot yesterday as (it looked to me) she was trying to lay her eggs. She's about to burst but she obviously knows he's going to be right behind her to eat them all up.
I really regret that I didn't watch her get "chummy" with another cherry barb but I never noticed anything going on. From what I just read online today - Cherry barbs chasing and getting chummy is only male/female behavior so it does sound like you have a male/female interaction going on. .