The whole "hinayana" vs "mahayana" argument came up at an historical point in time, so I (and quite a few others) thought that "mainstream" was appropriate for the non-Mahayana groups at that time. As you say, that may not work all the time. How are we do deal with this now?
Also, some of those schools are no "earlier" than the Mahayana (eg. a lot of Theravada theory - the "teachings of the Elders" is about co-temporal with the start of the Mahayana), so "early Buddhist schools" won't work either.
So, having rejected "hinayana" (obviously!) and "mainstream" as terms to describe non-Mahayana groups as a whole, do you have any other terms that will work? I think that this is an important question. If we are keen to drop the nasty terms as long gone history, how are we to proceed in the present?
My recently moved Blog, containing some of my writings on the Buddha Dhamma, as well as a number of translations from classical Buddhist texts and modern authors, liturgy, etc.: .