If I save before I kill Erandur, take the skull, and load to the previous save and spare him will it count for the oblivion walker achievement ?
Thanks in advance!

yes

edit: I did the same thing when getting married and then much later on found the achievement gone. Maybe Steam caused a "recount"
at some point? If it's that important to you, go for the trophy

Only if you already have 14 Daedric arifacts and the Skull of Corruption is the last one you get.