Can a Faction character unlock all heroes?

answered tyvm

You can get all heroes no matter where you character is from.

Yep, it doesn't matter where your character is born, as long as you own the campaign you can grab all the heroes from that campaign. Golems perhaps may be excluded, but that's another topic for another day.

Actually your Canthan native will have a slight advantage. Since you'll skip most of the istan island storyline, you'll pick up the heroes you would have acquired doing the story as soon as you step of the boat in Kamadan.

And they'll already be Lv15 as you get them.

Except for Souseke or Jin, whichever you pick, they'll only be lvl 11 iirc.

You get all but the EoTN heroes, unless you also have it one your account. In fact, upon traveling over from Proph/Factions, you get 4 lvl15 heroes from the first quest and a 5th lvl11(or is it 12?) hero with another early quest. You'll be able to gain the remaining heroes in the same sequence as a NF-born character.

This is correct. You will also be able to get any heroes you missed (because you'll run into situations where you can only pick one) once you beat Nightfall.