You're using metagame info of Teagan and Eamon surviving. You're also using the odd excuse of "but the warden might die!" when this applies for every single main quest in the game.
Yes, but those side quests never offered a similar chance for you to simply not participate in them if they required risking your hide and find other means because video game. RP-wise, they very well should have, and I'm sure both Sten and Morrigan would have similar stances on most of those nonexistent choices. This is especially 'gamey' in the Broken Circle quest where you even have an option to let the Templars carry out the Right of Annulment once the reinforcements arrive from Denerim... but you must clean out the Tower first and do their job for them all on your own anyways. There's no way to help them carry out the Annulment on their side or just wait for the reinforcements to arrive with some penalties to other subplots because of time loss.
On the other hand, you can strike a deal with the demon in Redcliffe instead of fighting her at all costs. That is a nice RP touch.
Some allies are clearly necessary though, but every potential ally is in the middle of a violent conflict (something that the characters also don't know until they reach each one on foot due to the absence of ThedasNet). I'm curious which poison they would pick if they did know about this quest design but we'll never know now.
This is a combat-based RPG, so the player character needs a certain minimum of XP to proceed, therefore the designers force you to participate in those battles, even if it doesn't make the most sense storywise. It's a good enough compromise for me of course, but I sure wish there were more "tricks" to avoid battles through the game, like the one with that Desire Demon.
Teagan withholds info to get you to be more likely to help yes so does Morrigan.
Morrigan withholding information about either her DR intentions or the Flemeth thing (that have nothing to do with the Redcliffe arc) doesn't make Teagan less of a manipulative jerk.
As for the more people pick nice choices there's plenty of quotes from the devs and here's a mass effect stat.
Aside from that being a Mass Effect stat, even BioWare doesn't have the entire gaming population's history in their notepads even now in the always-connected age, just a portion of them at most. Non-BSN people won't be in there, pirates won't be there, people who played offline won't be in there (which is many many players) etc. I consider myself a core gamer, played all Bioware games so far, but I sure as hell have never been asked about my choice history by the devs before the DA Keep, nor could they have possibly fished it from me through the net.
Now that they have the Keep, I can see them using those stats in their future character designs, even though I wish they wouldn't do so and just continue to design their creations as they see the best (I'm generally opposed to fan service and have a big issue with Telltale games as a result).
At the time of DAO, up to 2009 (alas at the time of the Morrigan-behaviour-design period), not only did they not have such a method, I doubt they would have done so even if they could.
Let's say you're right and about 70% of all the players of the world were goodies and 30% of them baddies. Would this justify them tweaking Morrigan's behaviour towards neutral good (or neutral whatever), even if that 30% would enjoy it a lot more otherwise? Never mind that even goodie characters could do the DR with her without moral consequences, or some goodies brought her along simply for entartainment.
How is it that flat out evil characters like HK-47 (or Morrigan herself to an extent) are considered fan favorites then?
As for Morrigan it's not schizo unless you compare it to her past game behavior which would be a non issue.
I honestly don't get what you mean here. We are still talking about her person within the same game. The Warden, Alistair and the player meet her at the same time, at which point she already acts a certain way. There is no past beyond that.