1. having whatever sense doesn't mean he don't lie to you
2. there is no evidence the mirror is tainted via Darkspawn, the mirror is Eluvian anyway and Duncan is wrong or lie about it's origin
3. No, Darkspawn never use Eluvian
4. The Joining making you sick, it is like injecting yourself with a substence that contain a virus but the virus act slowly than normally
5. No ghouls work with Darkspawn, they can hear the calling but they don't serve the Archdemon, for exmple Ruck. And Duncan lie about he being dead.
6. Merill can cure herself from the whatever sickness the mirror caused using Blood Magic, since she's the Keeper's First, surely she use the same method Merethari used to cure Mahariel. That's the whole point.
This is not head canon, this is canon.
Did you even bother to listen to character conversations? Or do you just skip through?
1. Why should Duncan lie to you? If he wanted you so badly as a GW, he just had to conscript you. It's a friggin' blight, he can do that without bothering with lies.
2. Yes there is evidence, Tamlen is living proof, Duncans darkspawn sense is proof
3. They don't have to use it, they just have to be near it for a while to taint it - look, what happens to lands, where DS roamed for a longer period. Look how Ostagar looked. Look how animals changed, who lived near DS for a while (the spiders in the deep roads for example, the even are called tainted spiders or corrupted spiders - what do you think, that means?)
4. The Joining didn't make Mahariel sick, becauce s/he was already sick. The Joining gives you extra powers against DS for a while and kills you then, it's not, what you call a normal sickness
5. You become a ghoul before you become a darkspawn. Ruck will at one point change compeltely. That is the whole point: Tamlen tells you, that he don't want to become a darkspawn, because he has enough of himself left, to recognise, what is going to happen to him. He begs you to kill, so he wont become a monste - he tells you that, you just have to listen.
6. Merrill and Marethari can cure no living being from the taint. Merrill never claimed that nor is it shown anywhere in the whole game series. Merrill cleansed the mirror with the help of a demon, she herself summoned (not Marethari as you claimed before).
And obviously you can't decide, if the mirror is tainted or not. In the Dalish origin you claim, it's not tainted, but for Merrill to clean it, it is suddenly tainted 
And as for your claim, that the Dalish has no reason to join the Wardens: The Dalish has even more reason to join, than any other origin: the taint not only threatens your life (it's not a little sickness, like you said, it's a death sentence), he/she gets a first hand experience, what will happen, if the Blight goes unchecked and gives the character (if he/she is not a totally selfish bastard) a reason t o help stop it. The only other origin, who could claim that, is the dwarf noble: you fight darkspawn too in that origin and get a glimpse in what could happen.