With 主にチョコを食べる, first off, this sentence is actually ambiguous in meaning: is it a response to 「主に何を食べる?」 or 「主に何をする?」? In both cases, I think 主に is *not an event modifier*. I don't think Neo-Davidsonian semantics can represent it, or at least I don't see how to do it. It's something like "for a given event e, e is predominantly a chocolate eating event" vs "for a given eating event e, e's theme is predominantly chocolate".