Either that or the Church and mages just retroactively covered it up, given the chaos at the time and the fact that methods of spreading information were far more limited back then (the printing press wouldn't have existed back then and books would have only been available to a handful of upper society as a result) it's very possible they could have covered it up.
Funny that you actually mention nidhogg, one of the other ideas I toyed with in terms of a singularity based on 536 was the idea that the reason the world tree even exists is that Odin purposely grew it to keep Nidhogg locked in by the roots of the world tree with Niddhog actually being an entity called Ditheader (Di-eatha-deer [Short for “itheader dia” or “god eater”]), I basically had the idea that Niddhogg was a bit of the White Titan's corpse which still was alive and somewhat sentient, divided from the main body and tried to keep devouring the earth. The gods locked it underground then Odin planted a tree on it, however come the end of the age of the gods, the roots of the world tree rotting and such it started to awaken and try to rise. Eventually using the magma vents and chambers beneath Iceland it arose and ended up fighting with the remnants of the British / Celtic / Welsh divine spirits. I also considered incorporating the theory of the continent of Icelandia into the tale (and also valid some older Celtic or Irish legends). Basically some scientists believe Iceland may have used to be much bigger and what we see today is the remnants of a sunken continent which may have connected to Greenland and quite possibly the rest of the European continental plate (their still would have been some bits of sea between them, but those part of the sea are situated above the edges of the continental plates).
Basically I would argue that the scientists were right, but they got the date wrong, Icelandia was a content, but it didn't sink ~10 million years ago, it sunk around 1500 years ago when Nidhogg (and Nidhogg even awakening to begin with may have been a plot by Gaia to deal with humanity) and the gods had their epic battle then the mages / the church / the world all covered that up in the aftermath.
I also had some other ideas related to Donn, but they never were that interesting.
I don't know enough to say if it would be possible at technical level in canon Nasuverse, as mages can be all over the place in terms of what is or is not technically possible for them. However I would be tempted to no in that I think Typhoon similar to the bull of heaven and Moby Dick / Leviathan was desired more as a "super-weapon" or living weapon than an actual living being and I'm not even certain if it would be recorded in the throne of heroes or not. Plus it's entirely questionable either if Typhoon loves humanity (a requirement for all beasts) or is even sapient enough to qualify as such. Beyond that I would think that the age of the gods ending may have prevented such actions as I think after the age of the gods ended for greek myth the greek underworld went with it, so depending on how you choose to interpret it (in that Typhoon may be in a part of the underworld geolocated under Mt. Etna) he may have either vanished with the greek underworld (or went wherever that went). Alternatively if you go by the Celtic explanation where the underworld is likely either connected to or a part of the reverse side of the world, he may be stuck there. Either way summoning him would I would suspect would likely face major pushback both from Magus bodies and possibly Gaia / Alaya's counterforce due to the amount of damage it would may cause.
However there is a loophole to this that I could conevieve of a well connected and powerful mage or magus group abusing, bear with me on this one, I could conceive of a ritual, albeit a very large one (on the scale of a holy grail ritual) to use Mt. Etna itself (as in the literal mountain and physical location) as a vessel to summon Typhoon. You see in Greek and Roman myths there is the idea of a Genius Loci which regionally had the meaning of a nature god ruling over a local place, however it later became or turned into the idea of a specific place as a god (similar to shinto belief that places or objects can have spirits). This meshes with the idea of nature spirits and assuming Mt. Etna has a nature spirit it as well as the physical mass of the mountain could be used as a vessel to summon a being similar to Typhoon (either with power derived from Typhoon's myth or possibly holding Typhoon's mind and spirit in the empowered nature spirit with the Mountain itself being literally used possibly as material for a giant Golem [think Adam from Fate: Apocrypha] to basically mimic the effect of truly summoning Typhoon, while bypassing the logistical hurdles of such). As NINE, mentions the lack of mana would be a problem however unless you used an explanation like a genius loci / nature spirit which would get energy from the world as a spirit to fuel itself (although I may be getting that incorrect).
I will also give credit where it's due part of this idea was based on a sheet based on Mt. Vesuvius as a servant that I distantly recall reviewing.