Metal in early 70s was basically riff-driven music with long and elaborate melodies. There's not a single Queen song that fits 100% in that category (or any), but of course there are similitudes. Liar for example, has many of those elements, but it's also related somehow to progressive rock (especially with the acoustic guitars), same for Great King Rat. Modern Times RnR is more proto-punk than speed metal imo. Ogre Battle may have a metal intro (for those days standards), nowadays it'd be just considered "hard rock". Same for some other songs (Son & Daughter, Stone Cold Crazy).
Proto-punk would probably be punk before punk became known as punk, basically prototype punk. And I think you're right about music being over-categorized. That's what I love about Queen; their effortless ability to jump from rock to pop to folk and then back again. But you know music has definitely been over-categorized when porn groove is listed as a genre!