> timedelta / <float or int> should be disallowed in true division mode.

I don't understand this;  why should the division mode affect
division operations involving timedeltas at all?  The meaning
of "/" is unaffected by the division mode for float/float or
float/int;  why should timedeltas be any different?

I vote +1 for timedelta/timedelta and timedelta/float (regardless
of division mode).  timedelta / timedelta is the one obvious way
to find out 'how many A's in B', and the one that it's natural
to try first, before looking for (timedelta -> float) conversion
methods.