I wrote a rather lengthy analysis of this in the German forums. Apart from what you've already mentioned, here's the abridged version of the stuff I additionally saw in the story:
On the the player and NPCs:
- Jespar, Player Character: Guilt-ridden over being unable to save someone close to him from danger
- Calia: Has an inner demon implanted into him by a foreign force (his father, via his superstition about demons)
- Player Character: The Butcher is seem as a "tainted" child because his mother is a prostitute. The player character is seen as a "tainted" child because s/he was born from bedlock (there's still debates on whether or not the tainted child actually was the PC's little sister).
- (New) Tealor: Both want to save the world from doom, even if this comes at the cost of others
On the veiled woman:
- If the Butcher saw her, but also is delusional, then it could be deduced that the PC also must be delusional, or the Butcher must be sane.
On the Butcher being Fleshless
- The initiation ritual: The Butcher had to ritually kill himself in order to become part of the order, but someone felt just fine afterwards. Seems similar to a certain someone who drowned (PC) and a certain someone who froze to death (Tealor).
- As you've rightfully stated, the Butcher the speed at which the Butcher learns new skills mirrors that of the PC
- When the Butcher realizes that his last victim only became what he was because he previously had murdered his father (pay attention to the bear emblem), he quickly realizes that he does not remove the corrupt, but rather creates a cycle of murder and cause for murder. Qualian outright tells him that it is a cycle. Like the bigger cycle we know from the main game, where the High Ones merrily influence the foolish mortals to their own demise and misery. Pretty much the most obvious connection.
The Nectar of Sin (Unsure of the correct word for the English version of the text)
- Mirror's the PC's ability to look in the past, especially towards violent events - even if on a much smaller scale, and with other triggers
On Mitumial Dal'Joul:
Mitumial actually mirrors the character of Jael - both suffer from an abusive father who only sees weakness and corruption everywhere, and who mistreats their respective mothers. Unlike Jael, however, Mitumial realizes that the behavior of his father is not him being plainly evil, but him being weak and helpless. Mitumial originally refused to become like his father, but tragically, the latter's death at the hands of Jael seal Dal'Joul's fate and turn him into an equally sociopathic person as his father was and the Butcher is.
On the Idea that the Butcher is bonkers:
Giving the addendum in the tenth book, it seems that the Butcher is completely delusional, murdering the homeless and the sick as he imagines himself as some sort of Vigilante. However, the PC in the main game can find a weapon called "Qualian's Salvation" (again, not sure if this is the correct English name), a two-handed warhammer bearing the name of a character that, technically speaking, should not even exist outside of Jael's head. Furthermore, there's the entire thing with the veiled woman.
So, the Keeper who wrote the addendum seems to have done the Butcher quite some injustice, but as the OP has also rightfully started, there's also a fairly reasonable chance that the PC is as bonkers as Jael is, and in particular, that the entire events of Enderal are nothing more than the PC's "Light at the end of the tunnel"-scenario as s/he drowns after being thrown overboard.
The latter theory is too big to go into to much detail here, but consider that the most important NPCs (Jespar, Calia, Ryneus, Tealor, the Aged Man) all somehow mirror the PC's fears and ambitions, that the entire Fleshless/High Ones plot could just be a metaphor for the PC seeking absolution in his last dying breaths (or the lack thereof), and that the Veiled Lady only shows the PC "What could have been".