I think all that was needed was someone to draw that little design on the floor and stand at the tip to bind themselves. Blood of Daedra, was just to send you on a silly mission and distract you with the tough decision of what artifact you want to give up. Blood of the Divine was just their way of telling you to go and clean out a crypt they've been too lazy to deal with. Greater Welkynd stone was because it looks pretty. Greater Sigil Stone was just because the blades were hoping that in the destruction of Bruma they could help themselves to some of the Akaviri equipment they love so much.
When you look at it like that, Mankar didn't need any help, and you were just being kept busy while they were deciding what to take from Bruma first. On the plus side, it makes you feel important. When you look at it, most quests could be looked at like this. It's just stuff they don't feel like doing themselves, or something to keep you or other people distracted from what's really happening.
If you remember, Martin used the Great Stones on the sides, and they only had you get the artifacts to figure out what to do with them when you don't have them, and the Blades want the armor of tiber septim to hide somewhere and worship it secretly. The armor Martin wears is Emporer's Armor, because after doing some searching in the CS while making a cheater's mod for myself, I noticed that there is un-playable emporers armor, and made that playable as well as Watch and Palace armor. The Great Stones are the real things that are used to open the gate. I bet Martin and Jauffre were confused, so they sent you/me/everyone else to get that stuff and give it to them, which is a plus for them, and you're going around to four places that are far away from other places to get stuff that only half they'll need. I'm very dissapointed in you, Martin

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