The Tiber Septim, Paarthunax and the Blades bit does confuse things.
If Tiber Septim parlayed with Paarthanax, and let him live, and declared him off limits to the Blades, why do the current two Blades not respect that original decision and make contrary demands of us? The player is simply following in Tiber's footsteps by seeking the dragon's advise and then just letting him be.
Because she's Delphine?
The answer died with the release of the game.
EDIT:
i thought the dragon guards came into skyrim because they were chasing the fleeing dragons, and because they were in a new land they had alot of clashes and destroyed alot of villages/etc until the dragonborn of that time showed up and they fought him until he shouted at them and they realized what eh was and surrendered and placed themselves into his service under oath as a way of serving the dragonslayer AND to make up for the deeds they have wretched upon the land zeosly chasing after the dragons.
There were no longer any dragons for them to actually kill. Every single dragon in Akavir was killed during the war between Tsaesci and Po Tun
The only reason the Akaviri Army came to Tamriel was for crap and giggles (that is, to do what Aldmeri Dominion tried to do back then, conquer Tamriel). That is, up until now, when Reman Cyrodiil suddenly can use Thu'um