How plausible is the theory that Snorri re-wrote Loki into a Lucifer-like enemy of the gods in order to keep the Norse stories alive in a Christian culture, but Loki was more of a positive trickster in the original mythology?

(noone else going to answer this? I'll give it a shot)

Nothing is really agreed upon about Loki, including the idea that he'd be a 'trickster', which due to Jan de Vries ( The Problem of Loki ). The idea he could be connected to Lucifer go even farther back to the early 19th century and Jacob Grimm, who made an etymological connection to fire (like German lohe ) which is not really believed anymore. In the late 19th century Bugge went farther in drawing the connection in his attempt to find Christian origins for all the Norse gods. Anna Birgitta Rooth connected him to being a spider via Old Swedish 'lokke' which means that. Or they could be formed from the same verb (lokka, 'to lure') and therefore mean 'one who lures'. Another possibility that's been pointed out is that he could've had something to do with the Celtic god Lugus The Edda stories are divergent, with Loki sometimes being good, sometimes outright evil, sometimes no really playing much of a role at all, not to mention changing species and gender. It's hard to give him any definite attributes.

So don't know where he came from. Unlike other gods, especially Tyr, Odin, Thor and Freyr, there doesn't seem to have been a cult around him. He's got no weekday (some have wanted to give him Saturday, which is 'wash day' in Scandinavian, but that requires another re-interpretation), and more importantly there aren't any place names named after him. (whereas the former gods have Tislund, Odenslund, Torslunda, Frölunda, just to mention some with the -lund, 'grove' ending)

Now, Loki's most malicious act, of instigating the killing of Baldr, is in Snorri's Edda (Gylfaginning). That story has been singled out more than any other as being a Christian influence. The parts that remain in the older Poetic Edda is Baldr's funeral ship getting stuck, and Loki being punished (in Lokasenna) but not for Baldr but just for being a dick to everyone. Also absent is the statement that he would be bound until Ragnarök. So Loki's still a dick, still gets punished, but not quite as evil. (nor is Baldr quite as good) Also the skaldic poem Haustlong mentions Loki as being bound and hated by other gods.

Now the whole binding-evil-'till-doomsday has fairly obvious parallels to biblical fallen angels (2 Peter 2:4, Jude 6). But this was not necessarily invented by Snorri. Besides the binding itself being there in the older version, an interesting artifact is the the 10th century Gosforth cross from England.

This is a Christian cross, with the crucifixion scene on it, but it (and a related stone) also have scenes from the stories in the Poetic Edda (it may be older than the actual text), including what's believed to be the bound Loki, and Thor's fishing (from Húsdrápa), which some have also made Christian parallels to with the Leviathan in the book of Job. (Interestingly, the later Stockholm Homily Book glosses the word 'leviapan' as 'miþgarþsormr')

The same connections exist on Scandinavian runestones from the 11th century. Runestone U1161 in Altuna has Thor's fishing on it (including Thor's foot through the bottom of the boat, a detail only found in Snorri's version). It's one of the stones signed by Balle, who otherwise produced many christian runestones in 11th century Uppland. Similarly, G113 from Gotland has both what's interpreted as Christian crosses and an image of the bound Loki. The 11th century Snaptun stone is also interpreted as having Loki on it.

Interpretations of this vary, one being that this would be didactic; explaining Christianity in terms of similar earlier beliefs, but another is of course that it's just straight-up syncretism.

But again, very very little is really known for certain about Norse beliefs. While Snorre may have embellished Loki's satanic qualities, it's not really possible to exclude that he was entirely a product of Christian syncretism either since he can't be dated with certainty to before the 10th century, at earliest the 9th, where he's already popping up in contexts where Christianity is present. On the other hand, the quite syncretistic 12th century Nidrstigningar Saga paints Satan variously as a giant (jotunn), a king of devils and trolls, and as the Midgard serpent (reference to Revelation 12:9 possibly), so not Loki but perhaps his kin.

Anyway, I can't pretend I'm an expert here. There are gazillions of interpretations of Loki outside the 4-5 I mentioned, it's a whole sea you can dive into. But summarizing the specific points: Was Loki a worse guy in Snorri's Edda than in the Poetic Edda? Yes, I think it'd be fair to say so. Was that a Christian influence? Many believe so. But it's not necessarily enough to expunge him completely from fallen-angel-ish qualities. Even him being caught in a net has been interpreted as a possible Christian influence. I'm not so sure, but at best he was a late addition to the pantheon, but there's really little outside the stories to indicate that there really was a functioning god named Loki, in the sense of someone actually venerated or worshiped in some way.

Anyhow, it's been a year or two, but last I saw Loki he was still bound up under city hall here in Stockholm.

Some more sources:

Stefanie von Schnurbein - The function of Loki in Snorri Sturluson's Edda

Yvonne S Bonnetain - Potentialities of Loki,