Simple fact C6 is not The dragonborn as such he can not trully kill a dragon and the world would be distroyed.
The dragonborn? What's to stop him picking up a plasma rifle? Think about it magic guns and shouts not to mention a pet dragon to summon one that once again can only be killed if the dragonborn absorbs it's soul.
And that's the kicker you need to send C6 with all this gear and inifinate ammo but the dragonborn you send him but naked and he'd still get the job done.
But saying that we'll go the infinite manna route with all shouts and all spells he can raise the dead for god sakes get enemies to kill each other for him
It's not even a close call courier 6 = takes over skyrim shortly after the world ends
Dragonborn takes over a post apocalyptic Desert and everyone bow's before him an new era is ushered in.
Plus I'm pretty sure all nords would fight against C6 as he isn't a nord so you'd have skyrim and the empire to deal with but the dragonborn?
Anyone of those piss ant little gangs would follow him kill Caeser bring him back as a perma zombie and bang the entire legion would now before you (or piss themselves and run)