Would Coutinho be cup-tied for the CL group stages if he played in this qualifier?
I think so, not 100% sure though, pretty certain he would be.
Just read that he could have played in this tie for Liverpool and in the CL group stages for Barca:
"Coutinho would not be cup-tied for the Champions League even if he played for Liverpool in the second-leg next week though, given Uefa allow players to represent different clubs between the qualifying rounds and the group stage."