All the Dragonborn has to do is have a massive army of dragons and have them invade the Summerset Isles.

Really?The bend will shout affects only one dragon.

I would say use Bend Will to get the Imperials and Stormcloaks to work together, or to convince the Thalmor to just off themselves

Wheres the fun in killing them all?

No, Dovahkiin opens College of the Thu'um, teaches his disciples Bend Will when they're ready, and they all take their own dragon. Dovahkiin flies in on Odahviing or Paarthunax while the others fly in on assorted dragons and they sweep over Tamriel, uniting it under the Dragonborn Empire.

How to Train Your Dragon Elder Scrolls style?

It only works on one dragon.

After you leave a bend will dragon, it flies away. How do you know that it still isn't under your command, just not with you at the moment?

It doesn't always fly away. Sometimes it will stay and attack you. It says "My service to you has ended, Thuri." and flies away/attacks.

Killing them all after making them do a massive can-can line whilst singing about how awesome Talos is.

Or just sit in a cave with a hundred potions and Unrelenting Force shouts and take out the whole Thalmor military.

I prefer calming them and individually slitting their throats in broad daylight, for everyone to see. Although I shouldn't be so nasty to them. They don't try to kill me when I fly around as a giant bat monster. Kind of ironic, if you think about it.

"We only accept High Elves, Bosmer, and flying bat monsters. The rest of you can go shove off."