These demons are only shown in gameplay however and there is no acknowledgement of demons following intelligent demons, so it should be taken with a grain of salt. Just Like abominations coming out of ground in DA2.
There are multiple acknowledgments of demons following more powerful/intelligent ones. DAO Broken circle: multiple characters mention the Sloth demons lietuenants and that you have to kill them in order to get to the sloth demon itslef.
The Nightmare mentions explicitly in dialogue with Iron bull that Bull would make a fabolous vessel for one of it's minnions.
The desire demon that posessess Connor explicity refers to the undead as her army.
The Pride demon posessing Uldred is obviously in some sort of leadership position over the other abominations in cutscens as well as gameplay.
There is mage-Templar war and civil war in Orlais, not to mention the only location "slowly dying" is Redcliff.
Uhm, no. If it was only a local problem in Redcliffe, why would Corypheus himself desperately want to go back in time and undo the breach?
Demons cannot venture far from the breach and it would take million years for demons to destroy anything.
The breach is everywhere during the bad future. It has pretty much replaced the sky. Demons don't have to venture far from the breach because it is already everywhere and it took them 2 years to destroy most of (at the very least southern) civilization not a million.
Demons would stand no chance against united armies, because they would never have the efficiency and order of them.
Yet they tend to always kick the ass of wathever armed forces they encounter, until our plot armored protagonist comes along and saves the day.
A spirit of faith took care of Nightmare, so he wasn't that strong at all. Nothing a few mages and Templars couldn't handle if they are not killing each other.
If by "took care of Nightmare" you mean "mildly inconvenienced it for about five minutes until it reappeared completely unscathed" then yes, a spirit of faith (which is one of the most powerful spirits to begin with) "took care of the Nightmare". It would eat a few mages and Templars for breakfast.