First of Morrigan has the ability to kill the arch demon
It requires a Grey Warden to implant a Grey Warden-blooded baby in her womb... considering the plan was Flemeth and what Flemeth is, that knowledge isn't exactly common.
secondly your companions never succumb to the darkspawn taint
Admittedly this one is a bit odd.
Thirdly You say they have this knowledge of how to kill the arch demon, where did it come from Duncan says the only thing that happens is that you can sense darkspawn and your dreams and thoughts are linked to those of the arch demon
Duncan didn't exactly live long enough to give you all the inner knowledge of how to Grey Warden. That's kind of the point of the entire quest... Alistair knows about the dark bits about the dreams and all that, but he's a fairly recent convert himself... certainly not long enough to know how to perform the Joining, or what -exactly- to do vs. an Archdaemon.
That's actually mentioned in the narrative, how you know you're supposed to rally the armies (because you have the treaties) but beyond that, knowledge of what to do with those armies is completely unknown to you and Alistair.
You clearly haven't beaten the game... or else you'd know this knowledge is eventually found, in the form of a visiting Grey Warden from Orlais that you rescue in Howe's mansion. Once you rescue him and recover his Grey Warden supply, everything you couldn't do before or didn't know how to do is suddenly available.
And yes, you DO do all of those things. You can enact a Joining, you find out How To Stop Blight.
Fourthly The ritual thing is not explained unless you wantloghain in your party now if I must believe you or did I just forget that part, If So sorry
you seem to agree with me mostly yet apear to agree to disagree for the heck of it or something?
I disagree about it being inconsistent
Inconsistent means that it breaks its own rules. Dragon Age pretty much doesn't. You're a brand new Grey Warden, being taught what little you know by another Grey Warden who is new at this... the only one with knowledge of what to do is killed early on. It would be inconsistent if you DID have all that knowledge straight away.
As for Morrigan knowing everything... well, that's Flemeth's entire plan. Flemeth is an abomination above other abominations. She's the sort of thing that makes Pride Demons crap their pants. She is an ancient ancient being, and mysteriously connected with the Old Gods of Tevinter. The Old Gods ARE the Archdaemons. Her plan isn't to save Ferelden, it's to de-darkspawnize an Old God. And Old Gods were never GOOD. Morrigan is simply her pawn in this, she's just the woman Flemeth happened to have available at the time. Of course Morrigan would have to be let in on the plan. Killing Flemeth doesn't change the plan, as Morrigan--despite her bitchy ways--usually at this point has some sort of caring for your little team, and doesn't want to watch the only friends she has in the world sacrifice themselves.
Of course... Witching Hour was supposed to explain wtf happened there...