Am I right in thinking that in these comparatives "niż" and "od" are interchangeable?
The only difference is that "niż" takes acc. case (biernik) and "od" takes gen. case (dopełniak) ?
Yes, I think you are right. but:
"od" takes dopełniacz/genitive (kogo? czego?)
"niż" takes mianownik/ nominative (kto?, co?)