I think this statement is a bit ambiguous. The Bridge II card is essentially the network card that allows your DS Jr. to connect via the Internet. The way you stated it, makes it seem as though you are referring to the Bridge II and the ethernet card as two separate devices, which they are not.
No, there is no buffering happening via the Bridge II/DS Jr. The Mconnect Control app (or Roon) interfaces with the Qobuz/Tidal (or other streaming service) server and the music is delivered from the Qobuz server to the DS Jr. directly via the Bridge II, but no buffering is happening. This is quite apparent if you have a mediocre internet connection as you will get skips/pauses/hiccups in the music stream.
Yes, the Mconnect Control app is connecting to the Bridge II via Wifi, but not directly. The Bridge II is connected to your network via ethernet (not Wifi) and the Mconnect Control app is able to communicate with the Bridge II via the UPnP/DLNA protocol. There is no direct Wifi connection between the Mconnect Control app and the Bridge II/DS Jr.
This is not accurate. Mconnect and Roon are interfacing with the Bridge II/DS Jr. in the exact same way. Some will claim one sounds better than the other but that is not due to the manner in which each of these control software suites connect to the Bridge II/DS Jr.
Some will argue “yes,” and others “no.” The fact of the matter is there are a ton of variables that can and will affect SQ above and beyond just Roon vs. Mconnect, so it is by no means a simple argument to pitch/win. That said, there are those that will prefer one over the other.