Do you have anything which is not circular reasoning to actually back up your view?
"People" were afraid of dremoras in prior installments. The game is changing. That too can change, or the playable dremora might have to earn the trust of the locals. (There are rpg possibilities there.)
Are you familiar with a Joss Whedon program called Angel? (It was a Buffy the Vampire Slayer spin-off). The title character is a vampire with a soul who fights the "bad guys" and is a hero. It can be done.
I find myself agreeing with you; It could be possible for events to have shaped the lore (in the past 200 years) such that dremora are not the hostile demons that we think of them as.
BUT, from the game-play perspective that you keep arguing, it simply wouldn't work out. Without a HUGE focus on explaining to returning players why dremora is a playable race in the beginning, you would find that people who have not actually lived through those 200 years of change have a preconceived notion of what a dremora is and how they act. For new players, it wouldn't be such a problem, but after the sales that oblivion made, and they support that Skyrim already has before release, I'd say most of the people who will play Skyrim have already played Oblivion (pure speculation there).
I completely understand both of your arguments, but I don't think that you could reasonably have them coming out as a playable character in TES:V. Perhaps, show the players in TES:V that dremora are nice now (or at least sane), and then you could get away with it in TES:VI. Otherwise, it's too jarring for anyone who payed attention to Oblivion and maybe even too much effort to explain.