... you can set a Pedal bass in BIAB ...
It's on the chord settings (right-click->Chord Settings, or Alt-F5).
I found the D/D and similar curious. In a purely notational sense, there's no need for it. but as a means explicitly to instruct the bass to just follow the bass line, it's kind-of reasonable, maybe even inevitable.
Track Settings and Actions->Options for selected track->simpler
(F7) might help. In my limited tries, it seemed to change bass to alternating just the root and the fifth, rather than arpeggiating the whole chord.