"Is it considered canon by all or most of Theravadan schools?"
A few Evidences on the Age of the Kathavatthu showed that an examination of some of the materials of
the K. V. and the Asokan edicts shows that the
compilation of the K. V. was made, at least in part,
somewhere in the reign of king Asoka.
Kathavatthu‘The Book of Disputed Points’, being a canonical book belonging to the Abhidharma Piṭaka of the Pāli Canon.
"How did the post-theravadan schools regard the texts?"
Below are quotes on the Kathavatthu from wiki: (*I added bold to show emphasis)
Each chapter contains questions and answers by means of which the most diverse views are presented, refuted and rejected. The form of the debates gives no identification of the participants, and does not step outside the debate to state explicitly which side is right.
The debates are understood by the tradition, followed by many scholars, as disputes between different schools of Buddhism.
However, L. S. Cousins, described by Professor Gombrich as the West's leading abhidhamma scholar, says:
"In spiritual traditions the world over, instructors have frequently employed apparent contradictions as part of their teaching method – perhaps to induce greater awareness in the pupil or to bring about a deeper and wider view of the subject in hand. The Pali Canon contains many explicit examples of such methods. (Indeed much of the Kathāvatthu makes better sense in these terms than as sectarian controversy.)"
Seen in this light it is not a book on points of controversies but a teaching manual!