So, when an elf dies, his spirit is called to the Halls of Mandos. I remember reading somewhere that the fëa may be able to resist Námo's call and remain in Middle-Earth. Wouldn't this be a possible explanation for Celebrimbor's existence as a wraith? But then again, my memory may be completely wrong since I haven't actually read the History of Middle-Earth yet.
The Elves learned from Manwë that each spirit was imperishable within the life of Arda and that it was their fate to inhabit Arda to its end. A fate separate from that of Men. Once bodiless (due to death of physical body) they were open to the direct instruction and command of the Valar. They were summoned to leave the places of their life and death to go to the Halls of Waiting. If they obeyed the summons, different opportunities awaited them, for only those who willingly go to Mandos may be re-born. This is a grace and comes from the power that Eru committed to the Valar for the ruling of Arda. It isn’t something that a spirit itself could do. Those who return, only do so after Mandos has spoken the doom of release and then Varda and Manwë bless this release.
A spirit cannot be brought to Mandos, only summoned. The summons may be refused and that was quite frequent in the early days of the Elves. However, it became less frequent when the influence of Melkor and his servant Sauron grew. A spirit in that case would flee in terror of the Darkness to any refuge, unless the spirit was already aligned to the Darkness. Those of the Elves who had become corrupted and refused the summons had little power to resist the counter-summons of Melkor and were thus enthralled by him.
The refusal of the summons is regarded as a sign of taint. There were many who wandered bodiless in the world, haunting hidden places they once knew and not all of these were of a kind nature or untainted by the Darkness. Tolkien regarded the bodiless as being subject to bitterness, envy and grievance. Some were enslaved by Melkor and did his work for him, and carried on such things even after his banishment. They lied, had little in the way of wisdom and to call on them was folly. Tolkien regarded the summoning of such spirits as a wickedness in the heart of the individual and that such practices were of Melkor’s and the necromancers in the service of Sauron.
Tolkien also made it clear that some of these bodiless spirits desired physical bodies, not via the judgment of Mandos, no, but via unlawful means. These wretched spirits pleaded for shelter, deceiving the unwary, and then if admitted, they sought to enslave their hosts and use their bodies for their own malicious purposes. Sauron did these things and he taught his followers to do the same.
This further strengthens the position regarding Talion and Celebrimbor (along with the allusion involving Merry in The Lord of the Rings). Of course, The Lord of the Rings is a primary text and that which I have detailed above is found only in the Histories and some scholars would argue that these hold less weight. Due to their nature and the manner in which Christopher Tolkien chose to omit some texts and not others (but that is an entirely different debate). I do not prescribe to that. The writers at Monolith have clearly done their research and what is presented in their version of the mythos is not beyond the realms of what Tolkien envisaged. In fact, it fits quite well within his ‘cycle’ as he described it.
Is Mandos the reason the Istari remained in Middle-Earth beyond the first defeat of Sauron? Were the events from the Hobbit & onward known to him?
All was known to him and the other Valar. It was they who sent the Istari after a council was held. It was the method they saw fit to use considering all that previously befell Arda was due to direct interference.
