Spirits/forbidden ones and demons question?

First, I should note that all demons are spirits, but we use the latter term to refer to benevolent or at least non-malicious Fade entities.

Second, every spirit has a kind of duality to them, with the exception of the really simple wisps. Depending on circumstances, spirits can become demons and vice versa. Solas mentions befriending demons who became spirits because he was free of expectations of what they should be. Likewise, his friend was turned into a demon by a group of mages in the Exalted Plains, and then reverted to a spirit if you broke the binding stones.

Now for your other questions:

Envy was a demon summoned to the physical world by Corypheus.

The Fade Rifts pull spirits from the Fade and transforms them into demons in order to defend them, assuming they weren’t demons already. Although a revenant is normally a possessed corpse, the Rift has twisted that spirit into that shape to defend it. The non-gameplay reasons why some Rifts favour certain demons to defend them are not clear.

The Wardens summoned demons under control of Nightmare.

Command is a normally benevolent spirit that just happened to slip through the Veil in Crestwood. The Rift didn’t summon it and she had no interest in possessing anything, so she never became a demon.

There were a number of harmless spirits in the area just hanging around. They were probably just curious to see what the big deal was about the physical world, and now that they’ve seen it, they’re probably going to leave terrible YELP reviews.

I thought Solas said the reason they were demons was the trauma of being pulled through. So they were benign spirits on the other side but being violently jerked through turned them. This happened simply because the area around the rift was very unstable. Obviously there might also have been demons who wanted to push through.

Where did you get the information that it is the Rift that was transforming the spirit in order to defend itself? I was not aware that the Rifts were sentient. They are simply tears in the Veil. Demons that deliberately push through may want to defend the tears to keep them open but that is not the same as the Rift causing them to do so.

I must admit I thought of it more as a vortex, so like a whirlpool they get sucked in. Then we can use the power to reverse the flow, so it sucks them back out again.

Well it might only seem that way because we missed the last belch and they come in waves rather than continuously. However, an alternative explanation is that the emotions of the party attract spirits/demons to them across the Veil but normally that is where they stay. However, as we approach a rift the spirit/demons following us get pulled through. Or they are simply attracted to the Inquisitor and it has the same effect. In Jaws of Hakkon the Augur tells that the spirits are fascinated by the Inquisitor, because of the Anchor which Cole tells us makes us glow to a spirit, so they follow us around, sensing the anchor through the Veil until they get too close to a rift and whoosh.

I just finished Trespasser and looked at the slides. The one for Cole has him returning to the Fade. He's not going because he doesn't like the material world but instead because he thinks that's where he's most needed. If Cole doesn't have a role in DA4 it would be a shame. He doesn't need to be a squadmates but should tie into the main storyline somehow.

Ah, interesting.  Still, spirit-y Cole sees the Fade as the place to be.  It means he's aware that he can be more useful there.