Not really. Usually, since the IC Throughline will be either in an External or Internal Domain, it does make sense that he'll be either Do-er or Be-er. It's usually recommended to do that. As for Problem-solving style, it isn't necessary to do it (both characters can be Linear for example) but might make your story more nuanced and their interactions more interesting.
Though I don't think MC and IC must be opposites in every way, they still do share the same kind of problems after all (MC in Universe and IC in Mind will both be stuck, one externally, the other internally).
I agree with you here, a lot of things aren't very much detailed and we're left with a lot of questions at the end of the Theory book. But I guess it is so complex it might be hard to just tell everything about it. And there's the forum to answer these questions anyway.