To pull out a small part of your paragraph, I think you're right in pointing out that it's not a spell seemingly in Janus' standard repertoire. That would cause it to fall into my theory of situational reality where he had control of the magic as it was cast and cast through him, but that Glenn's form was chosen or perhaps fated for him to wear by the same reality awareness that possibly created the gates for them to travel from era to era and place to place. Otherwise, it would be a decent dark spell to shapechange enemies with.
I do, also, think that they were limited by the technology they had to work with and could only have so many concepts able to be played through the technology, to catch Mauron's addition. There could be nuances to it that come into play that couldn't be conveyed in full.
Lavos's involvement is a decent suspicion, and even if it could be surmised that Lavos had greater matters to attend his attention to as his own entity, learning and growing, that it's entirely possible that he did, but would still only play a small part in the actual power behind the spell or its permanence so that it didnt take magic to maintain the form, which killing a magic user that cast it would suggest.
I do think that Lavos' death could trigger the event, but more in an unweaving of magic and spells that would be unweaved much like certain parts of Final Fantasy 8 with time travel involved as the vestiges involved would lost interest or wane for other reasons at the end of the major catalytic events that literally changed and rechanged reality from in the inside.
Time is a vastly complex concept filled with entities beyond those seen and labeled. Glenn, no matter his humanity or human aspect, became legend through being a Frog, not being a human, and that plays into it more than any of the other aspects. Pure story-telling as art mimics life as it mimics art again and tells the stories told and played out to the tune of eternal entertainment among the mortal/immortal instruments.