Regarding the demonology and Goetic manuals, it is clear to me that by the early 15th century most of what is the standard later contraptions of seals, diagrams, etc… was in place, so I think it is perfectly adequate. Note that I think also that since these manuals were more “transgressive” and possibly the ones concerned with non-demonic spirits (like spirits of the hours, directions, zodiac, angels et. al.) were more acceptable to the church in general, we may have a distorted view of the actual prevalence of these practices. For example, it is possible to do some statistics and compare, for example indexes of various medieval libraries, and note how many books included were (or might be) concerned with spirit/theurgic magic, and compare to the number of actual known surviving manuals and then extrapolate the number of nigromantic/demonological manuals by comparing the know surviving ones and multiplying by the ratio. I have a doctorate thesis on ars notoria by a french specialist that does just that, and posits that demonological treatises were probably as numerous as theurgic ones, they probably were just more the victims of book burnings and so on.
This is L’Ars notoria au Moyen Âge et à l’époque moderne. Étude d’une tradition de magie théurgique (XIIe-XVIIe siècle), by Julien VÉRONÈSE, 2004 – if you read french I can send the PDF to you if you wish. Also, I recommend Religion, Science, and the Transformations of Magic: Manuscripts of Magic 1300-1600, by Frank Klaassen, and I can’t resist quoting it (it took me a while to remember where this was, LOL): “The third section demonstrates that there is a higher level of continuity between medieval and sixteenth-century magic than has been previously recognized. But the continuity should not be traced to scholastic image magic, which practically vanishes from the collections of practising occultists. Rather, medieval ritual magic deserves our attention as it forms the overwhelming bulk of the magical texts in Sixteenth-century co11ections. (…) In all these senses practicing sixteenth-ceahiry occultists may be seen as extensions of the medieval ritual magic tradition…” (this is also available as a free PDF)
So I don’t think it would be out of order to have demonology et. al. as a part of 15th century “real” magic. If I have one criticism of Codex Superno, it is exactly that, that it seems to have this glaring missing part of the book, though I perfectly understand your points about not going in great depths about it.
In any case, each campaign is different and should be adapted by the GM. For me, the attraction of a more “realistic” system of magic, one that would be both more low-fantasy, as it were, and more complicated, flavorful, not as systematized, always had a lot to do with Elizabethan magic, John Dee, even the more modern Golden Dawn et. al. magic systems. I just think they are super cool. And since I don’t play a European campaign (I play in my own version of Greyhawk), I feel completely free to adapt, steal, change, add anything I want! 🙂