I think I know an easy way to make Shadow of Mordor canon without violating Tolkienesque lore too much. It is said that "Apart from Beren, no mortal man can return from death (Gandalf is clearly not a mortal man, so that doesn't count.).
My easy fix for that is this: The ritual that was supposed to summon Celebrimbor, thus allowing him to "revive" Talion, plus Celebrimbor's "interference" didn't revive him, it actually trapped them both between life and death, kind of like [] from Elder Scrolls V: Dawnguard.
I think it would require much, much, much more elaborate changes to make the games close to canon :). First of all the time period (so to not contradict the appendices which say that watch on the borders of Mordor ceased officially in year 1640 T.A.) then instead of Morannon the Black Gate and the Towers of the Teeth, which were held by gondorian garrisons still around the time of war with Wainriders according to The Unfinished Tales when king Ondoher died but soon was lost, instead set the game in Cirith Ungol around the time right before the great siege of Minas Ithil in year 2000 T.A (city was taken by Nazgul after two years in 2002 T.A.). Then we can have even much more richer story, since from book we know that: "treachery yielded the tower (of Cirith Ungol) to Lord of the Nazgul", we could have a whole subplot about the traitor who would help Nazgul take over the fortress and slaughter whole garrison, and so to chase him and all that, and still we could have the Black Numenorean captains and also Nazgul above them as bigger bad and that way get to sequel (Nazgul gathered in Mordor long before after the Great Plague emptied the forts, then in year 1980 T.A. Witch-king after succeding in destroying North kingdom of Arnor came to Mordor and gathered Nazgul under his command again). This way we could have a canon way to portray the siege of Minas Ithil as it 'really' happened.
Besides the game would require a sort of expanded prologue so we could interact with our family and fellow soldiers of garrison more (after all Tolkien fictional universe is all about story), maybe even doing some quests for them, going out fighting together you know so their deaths would have more meaning to us, a chance to bond with those characters to have greater emotional impact (the lack of it in first game was a serious flaw in my opinion, it was all so rushed). Cirith Ungol might be smaller outpost than huge fortress city of Minas Ithil but it was still very large and maybe it would do good to have some interiors to explore, more caves and tunnels.
As for the metaphysics of 'death', the only way would be if Talion was not really dead but simply in a state close to it, but not enough so his spirit would escape the body. Of course since The Unfinished Tales tell us specifically the fate of Celebrimbor and War of Elves and Sauron and sack of Eregion so there's no possible way that Sauron could have taken Celebrimbor to Mordor and all that flashback could not take place (the whole mess of stealing One Ring and waging war against Sauron is impossible due to time constraints as well as impossible because as Tolkien wrote "none but very few of equal stature could have hoped to withhold it(Ring) from him", besides it was Sauron who was master of the craft who taught the elves how to forge Rings of Power, he would never ever need the help of his student no matter how skilled an Elf it would be to 'perfect the Ring'). Celebrimbor simply has no chance due to his nature as an Elf to ever reach power level of a being like Sauron.
In general the state of Talion and Celebrimbor is more akin to the Barrow-wights, not Ringwraiths. The Barrow-wights though were spirits that possessed the corpses, long dead remains of Men buried in tombs, devoid of spirit, the wights seem to even have some sort of magic "dreadful spells of the Barrow-wights of which whispered tales spoke", so here's room for all sorts of interesting powers (besides there's also sorcery, black arts of necromancy and so on). It's interesting to note that while the license only encompasses The Hobbit/Lotr so The Unfinished Tales and The Silmarillion beyond it's limits, the main premise of an elvish spirit of a 'dead' Elf that refused summons to Halls of Mandos and wander in the world, possessing the living is in late Tolkien writings from The History of Middle-earth, Morgoth's Ring, and these materials are also outside license.
This is my "Simple Solutions for otherwise complicated problems" answer to your post:
Celebrimbor was already an accomplished smith before Sauron came to him, and as we saw in Shadow of Mordor Visions, Sauron (in fair form) gave him the tools he needed to shape the Rings of Power. How else was Celebrimbor able to craft the Three without Sauron's aid?
Second of all, Sauron's forging of the One, to me anyway, is similar to Gehn's own style of Writing. That is, Sauron's shortcuts and inherent evil succeeded in creating the One alerted all the Wise (Celebrimbor, Galadriel, etc.) to it's creation like lighting a bonfire at the top of a tower signals the start of a war. It also explains why Sauron needed Celebrimbor to perfect it, so that it's true power couldn't be revealed.
Third of all, Celebrimbor stole the One ring to use it against Sauron to free his family, but he didn't succeed. That is why the Valar punished Celebrimbor to roam the land of Mordor until he could put things right: because of his hand in creating the One, and his use of it.
Fourth of all, if the Bright Lord Expansion is any Guide, Celebrimbor came very close to Sauron in completing the Tests that the One set before him in order to gain more access to its power.
What do you think?