OnStep will never be in Focuser or Rotator, but in FoucserInteface, WeatherInterface, etc, as all of the non interfaces inherit the base classes and Interface classes. The exception is Telescope. (Though OnStepX doesn't always technically have that.) OnStep, I think, has the most Interface classes of all INDI drivers. And unfortunately, is also kind of a pain about multiple interfaces. (See the whole multiple focuser stuff that works but yeah.)
The rotator is gated by version, so versions 1,2,3 don't work. (3 may be made to work, but lacks commands to map it well, as I recall. There's not a derotator interface in INDI. It also detects it on startup, So, without looking much see if :rG# returns something if you don't see the focuser. That's so that it doesn't recheck it (and have to timeout) if not present. (Similar to mentioned above.)
Unfortunately the INDI interfaces doesn't map well to OnStep in a number of cases. (OnStep isn't very traditional in how it works, which makes it great but also sometimes a pain. Ex multiple focusers)
I used simulated hardware on a mega (with nothing connected) for most of my testing.
Alain, sorry I've been delayed on that pull request. I plan to work on it this morning. I'll look at the diffuser not showing up