a dog, Eleanor, and all the soldiers still alive in the castle. Do you seriously think that you could defeat two or three dozen Howe soldiers in a single fight with only your dog? Knowing the castle is irrelevant. When Duncan joins you are at the exit. First you'd have to sneak past Howe's main force, probably over a thousand men surrounding the castle. Once you do that you have make your way south, shaking pursuers who will be tracking you with mabari and horses. Duncan's something like a 20th level Rogue who is going to be able to remotely sense enemies before they get into eye shot. Without him you are going to die. I'm not kidding when I say that the reason why that is skipped over is because it is Duncan, not the player character who would have been the protagonist in an escort mission for a comparatively helpless temporary companion.
I don't agree with anything that you said as it doesn't make sense from A to Z.
Bryce reason to sell the youngest of the family to Duncan was not a deal made to allow the noble to escape from the castle but more likely to offer protection later by using the ranks of the Grey wardens.
At that point Duncan help to escape is useless as both dog and the future Warden and Eleanor are perfectly capable to escape and this was proved by the countless soldiers that they killed before to reach the passage of whom only the Cousland family know the secret,without the young noble it is Duncan who would have died in the castle,because he didn't' know the existence of the secret passage or how to use it you are making the reverse here and the function of the secret passage was to avoid the others soldiers outside of the castle.
So in order to thanks the family that saved him Duncan decided to poison the young Cousland and forced him into battle at Ostagar 3 minutes after the joining,when the young Warden was not even capable to lock the Archdemon's nightmares
(see i have my own theory regard to what happened on the top of the tower of Ishal when the Warden got defeated for the only time in the game by those arrows,when it was proved that so few arrows cannot defeat the character in game i like to think that the spasm of the Warden's white eyes on the cutscene of the tower of Ishal is the Archdemon's mind control nightmare effect who defeated the Warden not those 2 arrows)
,really i would have given the champion cup for satisfaction to that Hurlock alpha who beheaded him
If the origin is not selected the noble die before to reach the secret passage not later because
Regardless of what you do Duncan always try to recruit every Warden of each origin who happen to die before that he could met them not later because like you claim they were not protected by him.