I've got this too. Honestly is pretty ridiculous. I'm still suffering from the Blood on the Ice bug so can't get Hjerim on my main lvl 78 character either. You can't expect people to play your game for nigh on a year and then release a DLC with achievements tied to a character who is meant to be killed in a faction questline. It is definitely becoming Thane and completing the Jarls radiant quest prior to killing Helvard that causes this, as his disposition is redeemable according to numerous sources around the internet by becoming Thane or getting the Black Briar Mead after assassinating Helvard. Anyone who says otherwise is just stirring it up. I've messaged Bethesda twice, once with a link to this thread. You guys should too, hopefully it will be addressed in a patch. I've still got a pre-Hearthfire save too but unfortunately no pre-Helvard death save so if it comes down to that looks like this DLC has ruined my 100% GS for this.
100% Retail: CoD 2, Hitman:Blood Money, Bully, Just Cause, Assassins Creed, Gun, CoD 4, Oblivion (1250), Bourne Conspiracy, OFDR, Fallout 3 (1550), Halo 3:ODST, Godfather (1250), Godfather II, Qauntum of Solace, MW2,Assassins Creed 2, Viking, Fable II (1350), Borderlands (1250), The Saboteur, Lego Batman, Batman:AA, Bioshock (1100).
100% Arcade: Battlefield 1943, Zombie Apocolypse.