1) I never said that it can only be learned through demons, only that it is traditionally (read: commonly) learned from demons. There are exceptions to the rule, but they are extremely rare, so much so that a templar or anybody is going to stop and think, "hm, maybe they learned their magic from a book or something."
Which, by the way, is extremely stupid, because if all magic can be learned straight from books, then what is even the point of having a mentor?
2) Jowan's method involves killing someone to save another, as opposed to using lyrium and some mages. Yeah, sounds good to me.
Avernus' research and use of blood magic results in the tower being possessed by demons (and consequently, Demon Sophia), and also the Veil being torn open and only TEMPORARILY sealed. As soon as he kicks the bucket, that seal goes away, and it's only because of YOUR help that it gets fixed. Yeah, sounds good to me.
Merrill's eluvian would have been used by a demon to escape it's prison and possess her, except the keeper intervened, sealed it within herself, and then died as an abomination, all in order to save Merrill from that fate. Mm. Yeah, that sounds good to me.
I haven't played Legacy yet, so I can't speak for Malcolm.
Morrigan saves the Grey Warden's life, but also damns her son to being a schizophrenic and almost loses him to Flemmeth. As opposed to being a normal boy. Sounds good to me.
Phylacteries are responsible for the deaths of multiple mages who just want to live free lives, which is the entire reason the Mage/Templar War kicked off. Mm. Yeah, sounds good to me. Just because the Chantry does it, doesn't mean it's good or for good.
3) Not much is known about rift magic, because it's a completely new field that came about as a result of the Breach and all it's wonderful shenanigans. You can argue about that all you want, but there's no supporting evidence for it or against it. Similarly, I can't seem to find a whole lot of information about necromancy, other than it's creepy, weird, but uses wisps/minor spirits to reanimate corpses, similar to how a spirit warrior would use spirits.
4) Entropy doesn't take hold of their WILL, and just because effects are 'similar' or 'worse' doesn't mean that the processes and effects are exactly 1:1. I can burn someone's hand, or they can touch a hot pot, but that doesn't mean that both should be evaluated (morally) the same just because the results were the same.
5) You're putting words in my mouth, here.
I never said that you were more prone to possession, only implied that your overall magical ability is weakened (due to your ability to access the Fade being weakened). You'll eventually stop being able to dream, even, so who knows what happens to your soul when you're dead?
Also, Solas is someone who damned his own people in his attempt to save them from slavery, and is also starting a race war with his endgame being that there will be no more Veil, period, and all of humanity being destroyed in the process. Respectable people my foot, people in-universe who argue in favor of Blood Magic all seem to be entirely short-sighted and not thinking about anything thoroughly.