Harsh distortion is present when the unit is severely underpowered, for example when you have a power hungry something connected to the same hub (on your motherboard) like a charger and there isnt enough power for the ODAC, at least that's my guess. If it sounds great then it's perfectly OK.
Q1 - Any odac setup? perhaps. There were reports that when paired with an isolator like ADUM4160 (available in kits) and external power supply the sound improves, although these were only subjective impressions. Probably some high-end usb hubs provide isloation from PC power lines, and most (if not all) provide external power supply which may be less (or more) noisy than PC's, thus theoretically making a difference. On paper if the ODAC is working fine then you won't benefit from a hub.
Q2 - USB hub may add more noise if it's power supply is of very poor quality (very, very cheap). Again, theoretically, as long as its working ODAC should not be affected.
Q3 - see Q1
Q4 - As far as I understand the ferrite core filters some of the high frequency power supply noise, so if you have it just at the ODAC end of the cable and not between PC and hub it should still filter any power supply noise, be it from PC or hub. So no, theoretically you don't need a ferrite between your PC and hub, though it would do no harm either.
Overall you can assume that ODAC was designed to be highly immune to noisy power supply and just enjoy the music.