Endgame

Just how much of endgame is solo-able with henchies and heroes? Will I be able to complete every story-based mission on my own? What about these Elite Missions I've heard about? And the dungeons in EotN?

How much can I solo?

You can do 95% of the story easily with Heroes and Henchies.

The other 5% is missions like Thunderhead Keep or Vizunah Square, where a human group helps.

Elite missions you should really have at least one other player, so you don't need to take henchies.
Some dungeons can be H/H

Only parts that can't be completly H/H'd are those without the option of taking hench, such as DoA. They can however be completed in a 2 person and 6 hero team.

I think he's talking about GWEN.
In Gwen everything is Hero/Henchable so long as you have most skills for all professions unlocked (to give your heroes proper builds)
The G.O.L.E.M. quest is rather hard h/h, cause of teamsplit needed, but doable with a little concentration and thought. Most dungeons are also h/h'ble although it gets pretty boring. Slaver's Exile is real hard with h/h but still doable with a couple of consumables.