IMO Jolla would be wise to follow the guidance for an OpenSource implementation (your “scenario 1”) which was nicely evaluated per “VoLTE support in SailfishOS? - #235 by NodaUK” for “Ubuntu Touch” / UBports (which has a very similar architecture as SailfishOS): https://www.jeroenbaten.nl/UT-VoLTE-proposed-technical-architecture.pdf
E.g., your “scenario 3” was not positively evaluated in that analysis.
As there were a couple of choices to make (as documented back in July 2021) for UBports, IMO it does make sense trying to look up, how that story continued (further documentation and implementation in public repositories).