I saw it as a true regeneration because Eight was already dead, and he simply chose what sort of person he wanted his regeneration to be. Time Lord regenerations are normally random, and the Sisters of Parn had refined it to such a degree that the dying Time Lord could choose what he/she wanted to be. If Eight had left it up to random chance, then it's possible he would have simply regenerated into another Doctor who lacked the courage to intervene in the Time War.
So, he made the choice to regenerate into a Doctor who would end the Time War and save the universe.
In the end, Smith and Tennant's Doctors acknowledge Hurt as a true Doctor, and he regains his name and stands besides all the other Doctors.
You never know, Moffat or Big Finish might decide to make a spin-off series of audiobooks about the War Doctor, just like what happened with McGann's Eighth Doctor.
Yeah, the Doctor/Valeyard could possibly happen the way you describe.
It would be really interesting if Capaldi's Doctor had to face a darker version of himself.