Who has the ring in this scenario? Is it frodo? Because if all of LOTR is working together couldn't Frodo just stay invisible and then go around stabbing everybody since Sauron would not be a threat? or does Sauron now have the ring again?
When it comes to Gandalf, Sauron, and Saruman, they are completely immortal as I understand it - and now working together on the same side, or did I miss something? Their physical form can be destroyed, but Eru - the Zeus equivalent of Tolkien's mythos - can send them back to Middle Earth if their cause is righteous.
The LOTR trilogy has shown that preserving Middle Earth for its kind is considered a righteous cause for Eru, so all who are Maiar (kind of like demigods) - and all the Tolkien Wizards and Sauron fall into this category - cannot be permanently killed if Middle Earth is the battleground. Or did I miss something?
Let's not forget that both Elrond and Galadriel can help the LOTR-side in this - as can the Balrog since he's on team LOTR now. Btw, it seems a balrog is a type of Maia as well.