All of these traditions are based upon the comment relayed in Menachot 29b which says:
אמר רב יהודה אמר רב בשעה שעלה משה למרום מצאו להקב"ה שיושב וקושר כתרים לאותיות אמר לפניו רבש"ע מי מעכב על ידך אמר לו אדם אחד יש שעתיד להיות בסוף כמה דורות ועקיבא בן יוסף שמו שעתיד לדרוש על כל קוץ וקוץ תילין תילין של הלכות
§ Rav Yehuda says that Rav says: When Moses ascended on High, he found the Holy One, Blessed be He, sitting and tying crowns on the letters of the Torah. Moses said before God: Master of the Universe, who is preventing You from giving the Torah without these additions? God said to him: There is a man who is destined to be born after several generations, and Akiva ben Yosef is his name; he is destined to derive from each and every thorn of these crowns mounds upon mounds of halakhot. It is for his sake that the crowns must be added to the letters of the Torah.
These teachings of Rabbi Akiva were actually preserved and written down in a midrash called the letters and crowns of Rabbi Akiva. This midrash from Rabbi Akiva lists crowns for almost every letter in the Aleph-Beit and where in the text they are used and where specifically the crowns are to be placed. They are not used everywhere in the written Torah. In the places where they are added, it indicates a specific teaching or tradition.
This Midrash was also preserved by the Rokeach, Rabbi Eliezer of Garmiza (correctly pronounced Varmiza) in his book, Sefer Kiryat S'far (Book of the Border Town).
Like it explains in the introduction to the new re-issue mentioned below, this midrash is the source in numerous halachic texts on Sofrut like Ohr Zaruah, BaHag, Tur and Beit Yosef.
Unfortunately, only a small portion of those teachings remain with us today. But through the outstanding work of Zichron Aharon a much larger portion of that teaching has been republished.
So to recap your questions, 1) Where do we put the taggin on letters besides שעטנ״ז ג״ץ? See Midrash HaTagin.
2)Are there traditional places on (the roof of) the other letters to put the taggin? The short answer is yes, as taught by Rabbi Akiva in Midrash HaTagin.
- What about שעטנ״ז ג״ץ when there are more than three taggin? See the answer to number 2.
...and your stated preference, "I'd prefer prescriptive statements in the halachic literature.
The specific answers to all three questions are detailed in the entire midrash. Copying the text of the entire midrash here seems excessive, but you can see a degraded version of it from the Lemburg edition of Kiryat Sefer here.
Like it states in Menachot, these crowns are to be learned via Rabbi Akiva's teaching. They were added specifically to the letters of the Torah for him.
It would be much more productive to obtain a copy of the re-issue from Zichron Aharon Publishers.