I doubt its locker-room issues over the captaincy. That was a long time ago, in hockey terms. By the time he came back, O'Reilly was well entrenched and also, it's not as if Tarasenko wasn't there one day and suddenly appeared. He'd been around the team for months before coming back. If there was anything to work out, they had ample time to do that.
For whatever reason, the team's struggles began when he came back, though that was right around the time when Sundqvist got hurt and was done for the season. (They overlapped for four games.) I think if you get offers for Tarasenko, Armstrong has to listen, though, as I said in an earlier answer, the Blues are unlikely to get any offers for him that will dazzle them and trading him to trade him is probably a mistake. The Blues probably value him more highly than any other team does. (Though I never thought the Blues would get the return that they did for Lehtera or Berglund.) This season, Tarasenko may be just another guy, but he also does have an upside and next season could be more than just another guy. After all the shoulder operations, his best seasons are likely behind him.
I can very easily see Armstrong making a bold, shake-the-team-up trade and Tarasenko would have to be a candidate, but I just don't know what the return would be. Also, Tarasenko has a no-trade clause.