The last part would be difficult as Elenwen is essential
Spoiler because she is scripted to take part in both the Main Quest and a possible CW quest. The former would not be a problem since her part in the MQ is over before you meet Esbern and open Skyhaven, but her death prior to her part in the CW could possibly break that questline. Not only is the CW questline completely optional, but that particular part of it may or not happen depending on what the player does and when with regard to the way the CW and the MQ interact with each other. If there were a quest to kill her it would have to be set up so it could NOT to be given until after the player either finishes the MQ completely or plays through the CW to the point where her continued existence is unnecessary (and that would be almost the very end of the CW.)
Aside from that, anything that results in Delphine losing her self-appointed authority over what's left of the Blades would be okay in my book, so long as it's a decent storyline and not just an excuse to kill Thalmor, which, let's face it, are easy enough to find and kill without any help from a rebuilt Blades faction.
Well, the DLC can start after the main quest.
I also agree with this Blades DLC; like I've suguested in a mod idea on gamefaqs, you can tell the blades "I am the dragonborn and I order you to stand down!" when demanded to murder Paarthunax (who was protected by all previous dragonborns and emperors too) And Delphine and Esbern will relent and let go of their petty revenge.
I also think that the Dawnguard should have an option to recruit and station members at cities and towns, because. As a vampire hunter, I find it sad that we dont even bother to station guards who are skilled in killing vampires to protect the living. I recently went to Riften where 6 vampires attacked the crowd. I was unable to get in there to attack, as I risked striking someone, so I could do nothing but watch.