The war of the ring did take years to end. However this timeline is different from the original.  

The main problem to Gondor is Y/n, or as I named him Abner. Most if not all orcs don't use strategy and tactics.  Y/n on the other hand, uses tactics in battle and has a strategy for the war.  The only land connection from Mordor to Gondor is Osgiliath which means it's a hot spot for both armies.   

I put Eltariel in the book because it would make sense for her to fight in Gondor versus doing fuck all in Mordor.   Plus she knows that as long as Y/n is around, Gondor's defense is in danger. 

That's really it. I will answer any questions that will be asked in terms of lore or anything like that.