As to the second point, Nan Elmoth still fits the bill, with its pre-established proto-Girdle for him to adapt to his own purposes. And it's still outside the Girdle. How he knows about it would have to be established, of course.
Since this is meant to be an early Season 4 storyline, it will still be possible for us to tell the story of Eöl purchasing Nan Elmoth before the Fëanoreans move into East Beleriand (albeit after the sun rises). That would make it very possible to show Eöl resenting his new neighbors, neighbors who didn't bother to get Thingol's permission (really) before moving in.
We will have to work out Eöl's motives for moving into Beleriand at this point. We can't use a 'he's always been here' storyline very easily. We could try it...but it would probably look like we were tacking him into the story rather than incorporating him organically. We are already doing a 'surprise, they've always been there!' storyline with the petty-dwarves of Nargothrond, so I think we'd want to be cautious not to double up on that and make it look like ill-thought-out worldbuilding.
As for how he knows Sindarin...does he need to when he first arrives on screen? He was last seen storming out of a meeting at Cuivienen in Season 2 Episode 3, so if we bring him back in Season 4 Episode 1-2, we could have him conduct his meeting with Thingol via translator (even a dwarf?) and then just know Sindarin after he's established in Beleriand. Any ongoing translation needs would get tedious, but we might get away with a single scene of that.
It seems likely that Eöl's point of contact would be the dwarves of Belegost, rather than the Sindar of Beleriand. So, we'll have to create some back story for him that makes sense of all of this. Does he know Men? Does he know Dwarves? Why does he want to come to Beleriand now? We'll have to come up with answers to those questions that tie into the rest of the story.
Since this is meant to be an early Season 4 storyline, it will still be possible for us to tell the story of Eöl purchasing Nan Elmoth before the Fëanoreans move into East Beleriand (albeit after the sun rises). That would make it very possible to show Eöl resenting his new neighbors, neighbors who didn't bother to get Thingol's permission (really) before moving in.
We will have to work out Eöl's motives for moving into Beleriand at this point. We can't use a 'he's always been here' storyline very easily. We could try it...but it would probably look like we were tacking him into the story rather than incorporating him organically. We are already doing a 'surprise, they've always been there!' storyline with the petty-dwarves of Nargothrond, so I think we'd want to be cautious not to double up on that and make it look like ill-thought-out worldbuilding.
As for how he knows Sindarin...does he need to when he first arrives on screen? He was last seen storming out of a meeting at Cuivienen in Season 2 Episode 3, so if we bring him back in Season 4 Episode 1-2, we could have him conduct his meeting with Thingol via translator (even a dwarf?) and then just know Sindarin after he's established in Beleriand. Any ongoing translation needs would get tedious, but we might get away with a single scene of that.
It seems likely that Eöl's point of contact would be the dwarves of Belegost, rather than the Sindar of Beleriand. So, we'll have to create some back story for him that makes sense of all of this. Does he know Men? Does he know Dwarves? Why does he want to come to Beleriand now? We'll have to come up with answers to those questions that tie into the rest of the story.