On self-healing:
I run 5 Blood DKs for farming and questing. I MOW through mobs and usually stay above 95% health.

I'm lazy and use this Gnome Sequencer macro when farming/questing (scroll down to his v2)

https://wowlazymacros.com/forums/topic/bloodbeta-8-0x/

My DHs are still too low to really test your question on that front, though, running them in older dungeons, their self-healing seems to be a little bit less than it was before, or maybe it's just me.