that's up to Forgotten Realms/D&D. Disney protects usage Twi'Leks and other Star Wars thingies. Warner (whatever is its full name) protects Superman, Supergirl and Superwomanand rest.
Yes and no. Again you can't copyright names (nor ideas or concepts despite what they want people to believe), however "Man of Steel", "Caped Crusader", and "Superman", and both Superman logo/symbol and Batman's logo/symbol are trademarked. Hypothetically, you could create something that doesn't look remotely like the iconic look and call the hero a Superman. Copyright wise it would be fine. Trademark wise? Ehhh...
Same with Disney and Star Wars. A lot of the names are trademarked (such as Obi-wan Kenobi, Darth Vader, Luke Skywalker, Anakin Skywalker etc), so they will come after you because of that. Not certain has the other species' names been trademarked but Twi'lek isn't in the US at least, so there is nothing preventing creating alien species called Twi'lek as long as they don't look like the Star Wars one copyright wise that is. Again, is it a smart thing to do? Depends how much money you have to burn. But personally, I wouldn't bother since it would be a hassle to deal with it.
So the whole copyright in these cases is more about the visuals of the characters/species and/or combination of the names to them. Trademarks? Yeah, they have to protect them or they lose them.
So regarding the whole Guenwyhar thing, as long as it isn't that specific combination, it is fine copyright wise. Is it smart? The fact that you can do something doesn't mean it is a smart thing to do. There is still certain professional courtesy towards the original authors to consider. That and very few people have the money to fight it in the courts. It doesn't warm you that much if you go bankrupt over it despite being "right".