Agreed, Josh, ya gotta start where you are.
I think I follow you on just about everything you posted...let me offer a clarification on the open/close.
I think of open coordinated with the in breath and seperating the hands, to split uke. This would fit with the back bow mentioned in the article you quoted and the other things that go along with that.
I think of the close on the actual throw, the hands coming together, front knee bent, zanshin posture, breathing out. I don't mean these to be set in stone, but in general, this is what seems to occur with this particular waza.
re: Breath pressure: This pressure can be moved by will power (via dantien rotation.. when structure is established.. this I think is moving the "ki" to where it is needed). I think in tenchinage it needs to go both upward in the aiki-age arm and down in the aiki sage arm. This points to a splitting of your power upwards and downwards. I am not sure on that. Perhaps downward is more passive; just using body-weight and kokyu connection. I do not know.
Well, in my limited experience, the down hand is less passive, the higher hand more passive. But I am completely willing to entertain other ways of doing it for the sake of discussion.