It is indeed a real word; I don't deny that. But given that this word is used to denote so many varieties of philosophical positions, is it safe to say that a view is "realist", without indicating what sort of affliation to which sort of realism was being promoted? Sometimes, the label is just used to dismiss an argument, and given the many levels of meaning to "realism", the risk becomes disproportionately higher.
You're right - "materialist" would not be my first choice, since that term has already acquired too narrow and specific a meaning in the context of "tri-temporal materialism" for one particular branch of Buddhism. Ontology comes in as a close favourite for this reason, simply because of its historical association with metaphysics. As I understand the injunction against "sabbam atthi" and "sabbam natthi", I think SN 12.48 really brings to the fore why the Buddha advised against such extremes - it was pure speculation (ie metaphysical, at least in the Humean sense that I am accustomed to) and not experiential. That speculation, coupled with one of the 4 types of clinging, is what generates one of the nidanas of DO.
As to the capital "E" atthi, I'll have to channel Gombrich, since I can't actually recall any specific details being recorded in the Canon of how "sabbam atthi" was supposed to have been explained by the Buddha's peers. I think the Canon might have skipped the details on these "Atthi" metaphysics floating out there, since SN 12.15 is swift to zoom in on the problem of such a view - it generates clinging.
As to the inclusion of the kāmā in the "All" formula, the kāmā are specifically mentioned in SN 35.23.