... for another, I dislike the idea of making the user wait at all for a response to a single-click, even just half a second. (Maybe the delay could be "optimized" if you can figure out or control the maximum interval that defines the double-click.)

It seems that the interval is defined in pTk/tkBind.c as the NEARBY_MS macro. This macro is set to 500 milliseconds and not changeable in the API, so half a second is a good guess.