To be fair, The First wasn't a mage either, but he still used powerful magic that was gifted to him by The Mother. So just because we didn't personally see any magic employed by the First doesn't mean he wasn't capable of magic, as Darkspawn are by their nature a sort of magical being.
3-Why send a force strong enough to take the Keep if he wished for peace?
A lone Darkspawn approaching a fortress run by Grey Wardens and manned with soldiers to the core who still have memories of the Blight fresh in their minds?
Under those circumstances sending an army is more a measure of prudence then a mark against someone. Negotiations between powerful groups tend to have armies at their back.
I simply believe the Withered took his orders a bit too literally.
My belief is that The Architect sent the Withered with an army, telling him that more then likely the Wardens would view his approach as an assault, so the army was to keep him safe. And the Withered decided that he really only needed to approach the Warden-Commander, so he decided to launch a pre-emptive attack as he snuck inside the Keep through the Deep Roads. The Wardens sensed the attack at the last second and rallied the soldiers together, killing many Darkspawn.
And though he says later on that he should've anticipated we'd view their approach as an attack, I think that's the actual lie in this scenario. Perhaps he's not telling us that he did anticipate it because he fears that we might view him as complicit in the Withered's literal-minded idiocy.
So my belief is that yes, the Withered was sent as an emissary and the Architect isn't lying. But the Withered screwed things up.