Because they were actively participating in the fight, and Miraak even send one of his dragons to kill the Dovahkiin, implying that he think that his dragons are capable of fighting against end game Dovahkiin. Their souls were also powerful enough to restore Miraak's strength to keep fighting against the Dovahkiin. Those dragons are greater dovah based on their power.
Because the dragon's will wasn't anything beyond other greater dragons'. Dragons' unyielding nature is a common trait across their species. They should be comparable to each other unless one has a feat or specific mention of being above the rest.
There is no credible story of how dragons came to be. According to dremora that the College of Whispers have "questioned," they just were, and are. Eternal, immortal, unchanging, and unyielding [...]
And that's just game mechanic. Even Sahloknir (an early game dragon) is unaffected by Bend Will, and so are the fodder dragons in Skuldafn. Heck, even the dragons that canonically fought mid game Dovahkiin, and has no story are unaffected by Bend Will. Point is, aside from Alduin, we don't know which instance of resistance is game mechanic, and which is lore-based. You can argue that one is more logical than another, but it will all just be headcanon.
It's doesn't come from Durnehviir. All I said is that the 4-D soul comes from it being the source of power for Miraak, who is low 2-C. Being able to power a low 2-C attacks automatically makes his soul 4-D. And if Durnehviir's feat is applicable (which it's not), that would make Miraak and the Dovahkiin's soul resistances low 1-C.