I think that a lot of the relationship is played out in subtext. The chemistry between them has been palpable from the start and there has been a lot of lovely body language moments, really something in each episode. As for Jack, he has a smile that he only uses for Ianto. His body language relaxes when he is alone with him. Also he can't take his eyes of him, and does often gaze in quite a dreamy way.
Absolutely. I'm not campaigning for huge declarations of coupledom, all the things you say are true, and they're things I've seen, I just don't think they've been quite apparent ENOUGH to the audience as a whole.

an overlay of histrionics
LOL. Wordy McWordyword. It scares me that with the team as it stands, we're going to have more Jack/Gwen scenes rather than less, because the way John and Eve play off each other, it's just... eww. No. Bad OTT stuff.