Gabriel Lazel was one of the mages responsible for sealing away the realm, but it was sealed away before the University was formed (while he was a 'young man', according to the intro), so it was between approximately 25 and 30 years before the events of the RP.
However, it wasn't an invasion, per se, but the daemon realm itself was starting to seep into this set of dimensions, causing dangerous spatiotemporal anomalies and letting various of the realm's inhabitants through at certain points (hence why the mages had to seal it away in its totality). The realm isn't hell, either, it's another plane of existence with some equally otherworldly occupants, named after what they were most reminiscent of to humanity - whether or not the name they've been given is accurate is not known. On that note, no major religious text actually refers to demons living in hell, Dante's Inferno just made the idea famous.
Bear in mind that the government still keeps a fair bit of information regarding the daemon-realm impingement under wraps due to its sensitive nature (for example, data about how some of the 'weak points' in our reality behaved could be implemented by mages to replicate such a weak point).
Mainstream science calling daemons 'exotic forms of life' and the daemon realm 'at least some of the additional dimensions mandated by string theory', various new-age spiritualists calling them 'the transcended ones' and their realm 'a higher plane of existence', and theists calling the whole affair 'proof'. As with many great scientific discoveries, different people have different interpretations and few consider it reason to change their stance.
The Hebrew, Aramaic, Greek, and Arabic words for 'man', 'mankind', etc., used in the original texts can refer to either Pelajae or humans in this universe, due to the difference in historical contexts between our universe and the RP's one. The physiological status of Adam, Eve, Jesus, and other historical figures are not known, although there's always going to be the occasional racial-purity group who believes that they all belonged to the 'better' race.
Consider your question tactfully circumvented.