Question about Blackrapid Sport Breathe

Hello Susanne.
1) Yes, I usually wear a backpack over the camera strap. You simply have to "thread" the straps of the backpack or those of the camera strap accordingly, then it works fine and you can operate the camera with the backpack on.
2) This is only possible if the tripod plate itself has another tripod thread for further accessories. However, I have not seen this before and therefore I would say: No, it doesn't work!

2) Yes, there is, for example the ty-lp70 from sirui. (It's just a bit clunky.) Alternatively, there are also L-brackets that have such a second thread - I can recommend them without reservation.