I hate "proto-orthodox" because of its connotation that these churches were "orthodox" when in fact they weren't. In fact, many of the main leaders of this church were later designated as heretics (i.e. Tertullian, Origen). I also hate "mainstream" because it suggests that there was a mainstream and everyone else was divergent. I find "apostolic" to be the least onerous because it suggests that these churches rallied around the twelve apostles and believed that their doctrines came from them directly, and it doesn't have any negative connotations in regard to other forms of Christianity.
If anyone has a better term to suggest, I'm more than open to hear about it, because I haven't the foggiest clue how to get out of this terminological dilemma! Thanks Phil for bringing this up.