Okay, I've been thinking about this for a little while and it's going to sound crazy, so hear me out. I was thinking deeply about Jim and out of all the humans the Amulet of Daylight could choose, it chose Jim. In the episode "Unbecoming", Merlin tells him this because of his pure of heart, but then he tells him that he's destine to be more than the first human Trollhunter and then transforms him into a troll. I know it's obvious in the right light, but I have a theroy of why Merlin would really do this.
I've also been looking up some Autharian legends and some information began forming in my head. I looked up on the internet and found an enchantress called the Lady of the Lake, or Lady Lake as I'd like to call her (who, in a legend, was Merlin's lover). What if, during a time, they conceived a child together and a glimpse of the future predicts that the decendent of Lady Lake and the Myrddin Wylt will one day bring peace and balance to the universe itself, as well as possess the power to undo and control even the most powerful spells, like opening a forbidden dark realm that could start an apocolypse. Morgana, believing that this child could be the key to her bring never-ending chaos to the worlds, vows to find and use this child. Worried of the danger, Merlin hid the child so his former apprentice and her followers wouldn't find him or her.
The child grew up and begins a loving family of Lakes. After several hundred years, the Lakes still have no idea where they really came from, at least until the modern times. James Lake Sr. begins to study these "abilities" he began manifesting after his son, Jim Lake Jr., was born. Wanting some answers, he leaves Barbara and Jim to find out who he and his son really are.
After Merlin finally meets Jim in person, his prediction turned out to be correct as Lady Lake's (as well as his own) decendant has been chosen to wield the Merlin's Amulet of Daylight. Knowing that Morgana would use Jim if she discovered that he is a decendant of Lady Lake, Merlin decides to turn Jim into a troll to lead Morgana astray, knowing that while the affects are permanent for a human, Jim was never just a normal human to begin with. Plus, Jim is not so easily killed as we've discovered.
So to my theory point, Jim is a decendant of the Lady of the Lake (or Lady Lake) as well as Merlin himself, hence his surname. So the Amulet of Daylight chose him, due to him being a legacy of two powerful sorcerers of legend. It may still be possible for Jim to change back to human form since the elixir potion is permanet for humans, but what if Jim was never just an ordinary human boy to begin with? Or because of the magic that can undo the most powerful of spells, including the one he used to turn himself into a troll? I'm just saying that Jim was always human, he's just a legacy of powerful magic and has a hidden power only the wrong hands would want to use for their own selfish reasons. This could be a possible hidden plot twist for Wizards... Eh, who knows until it's confirmed?