What if Eren had been "utilizing" the ability before he was even a titan? Consider how persuasive and moving Eren was during the Trost invasion or even when he made his speech arguing with Jean? He pretty much goaded almost all of the 104th into the Survey Corps by just being himself. You had all of the top 10 change their minds about doing Military Police...join Survey Corps. Same thing as when he goaded the Titans to fight him right?
As for his new power? I really believe it was gonna happen sooner or later.
I do think this reveal makes a lot of sense in hindsight, but although I'll admit I'm excited about the possibility, I'm not sure I'm ready to believe he had that ability activated before now. (I will say though, I would love if it if he had unconsciously activated it in Trost, and it was because he wasn't consciously controlling it that the titan trio may have felt something was up with him, but weren't completely sure. That would definitely tie up some loose ends, if true.)
Are you implying eren is using the coordinate to influence humans? That seems a tad weird considering that when eren did use the coordinate none of the humans noticed it. I don't think eren could plausibly use that power to influence or control people to be honest. If that were the case then controlling titans would be the least of the coordinates abilities.