First, where is this information about Sepulchure? I can't remember ever reading anything about his human life.
In an old lore thread from before I joined the forums, one of the staff said that he was stronger than the Hero. There's also this excerpt from the Irismancer, which still holds true:
The Irismancer: Not long ago he was a hero who accomplished many great deeds. He had made many good friends in his adventuring, like you.
The Irismancer: Then he lost something that was dear to him. He felt that if he had more power, he would have been able to stop this event.
The Irismancer: In his obsessive search to become stronger he withdrew from his life and his friends. They tried to help him but he wouldn't let them.
The Irismancer: Then he discovered his cursed weapon, the Necrotic Blade of Doom.
The Irismancer: His weapons did make him stronger but poisoned his mind further. Then it began to poison his body.
The Irismancer: The armor that he wears, the armor of the Doom Knight...it actually wears him.
The Irismancer: He has become something else, bound to the darkness.
If even weaker Doom weapons like the twin blades, shadowreaper, and skull staff can corrupt the Hero canonically, think what the much stronger Necrotic Sword, and all encompassing armor would do. Yeah, the three canon ones took years of attuning and growth to reach the point where the corruption is noticeable in the hero, but with the armor and sword, it would probably become irreparable in a matter of weeks, maybe even just days. And the problem about using them is that they're also addictive. Victims want to keep using them.
Second, the problem of not having a trainer can be solved easily: create a trainer. Since we know Doom Weapons can be purified, maybe we find Seppy's old armor, purify it, and have Amadeus train us to use his skills in reverse. Solved.
And why would Amadeus be able to, or even want to train us? He hates us, remember? And he was little more than a puppet for his armor as well, not to mention he only has knowledge on Doom, not its opposite. The requirement for a trainer is that they're both capable of teaching us, and willing to as well. So, not solved.
And one last thing you're disregarding about making a canon alternative: The entire reason behind Doom Knight in the first place as a playable class. It's a reward for supporting DF via funding. A canon equivalent that is a "good" reskin, as you put it wouldn't be such, and thus would both undermine DmK's existence, and remove what is easily the largest source of funding for this game, as why bother paying for DmK, when you can get an equally powerful class in game for free? See the issue there?