Actually i don't think, that rebuiding the blades will change situation. Why would Db do that. Since there is no more Septim dynasty,
Serve and protect the Dragonborn, and fight alongside them. The Blades existed long before the Septims in one form... Taking on the role of Dragon hunters.
and all the knowledge and forts and artifacts of the blades are lost, how the hell an old man and woman are going to restore the power? Restore the traditions, and all that stuff. The only relic they have, is a damn rock with a prophesy.
Which turned out to be true, and whose information was instrumental to the plans that saved the known world from Alduin. The Blades will basically gain fame by proxy due to their connection with the Dragonborn, that might lead people to wanting to be a part of the group that once served alongside such a hero (especially if they spin it right). Plus, if they do take on the Dragon hunting role, there's plenty of people that might want to take that cause on and sign up.
And how are they going to fight dragons with those recruits? Also, Thalmor won't allow them to act freely. I gues this is just a way to defend themselves. Simple egoistic fear of being cought.
Depends on the quality and number of recruits. Dephine is a fighter, she can handle their combat training while Esbern fills them in on the history, lore and such about the order and the knowledge of how to deal with Dragons and other issues. You bring them a few hardy and loyal followers to start them off, whose to say that later they don't find and recruit more people themselves? There may be other blades still in hiding that weren't known about or thought dead. They might show up with a few additional bodies to add to the ranks.
Thalmor: Bit of a Problem there yes, but if you've sided with the Stormcloaks for ending the Civil War - Skyrim might now be the safest place for them to be. The present Thalmor will have bigger problems to deal with than hunting a few rogue enemies. The entire country may well be mounting an offense to get rid of the Dominion's agents.
If the Empire wins the war, then there will be a lot more problems, but they are in an extremely well defended area. So it would take a long time to oust them. Especially if you've built up a sizable force before anyone starts bothering with tracking them down again.
As for the Greybeards... as far as anyone attacking them: Same thing as the Blades, well defended and hard to reach area to begin with, and if it came down to it, I'm sure that Paarthanax and the Greybeards would use the voice to defend themselves... maybe not in direct, conflict devised ways - but altering weather, halting progress through force and other shouts that would hold and drive opposition back.
You can't mount an assault on somewhere you can't reach or see. You wouldn't even be able to get any equipment up to the hall at High Hrothgar. The steps hinder heavy hauling, and if worked around, there's one of the most ancient Dovah in the world living at the top, all he has to do is swoop down, grab the stuff and toss it back down the mountainside. That's if he even needs to get that close - you imagine him using the Unrelenting Force shout on a siege caravan on a mountainside? There'd be very little left of that crew.
That's without saying that perhaps after the events, the Greybeards do get a few new recruits to the order, and maybe a few Dragons take to Paarthanax and his teachings. So there may be more than four old men and one ancient Dragon to deal with.