I can't answer all the questions, but:

I have only added the Dreame product so far, but I don't really see any reason why my own wouldn't work, as the container is removable.
I haven't changed the teat bag yet, it lasts a relatively long time. But we don't run Dreamy McWischface that often either.
Additional information: Long hair gets easily caught in the rotating parts, so check them from time to time.
Carpets in general are not necessarily his strong point, we have one that is about 3cm high with pile (if that's what you call it), he almost never gets up there. On another carpet it occasionally loses the mops, I have also deactivated this. Furthermore, it can even happen with flat carpets that it folds up the corners...

On the whole, however, we are satisfied.