These are some good UX questions, none of which have a really clear answer.
I think most of your 'when should the fields update' type of questions, have already been solved in Google Translate web UI. There, you can only update the left-sided input field. So there becomes this simple flow of: 1) decide which language you're translated from. 2) edit that value. 3) the UI picks up that a keystroke has not occurred in the last X seconds 4). The right-hand side field updates.
That was kind of his point though. In order to deactivate one side you have to program that in the UI. Also, if you provide this kind of forced directionality to the two temperatures, you now how to make the units toggle on each side ( otherwise you can only convert from one fixed choice to the other - eg. C to F but not F to C ).
As soon as you start asking these questions, the UI requirements change.
I think most of your 'when should the fields update' type of questions, have already been solved in Google Translate web UI. There, you can only update the left-sided input field. So there becomes this simple flow of: 1) decide which language you're translated from. 2) edit that value. 3) the UI picks up that a keystroke has not occurred in the last X seconds 4). The right-hand side field updates.