Easy. Simon had a brother, sister, aunt, uncle, nephew, or niece, and THEY had kids.
OR OR OR OR Desmond is actually Simon's younger brother.
-Simon's and Desmond's father dies, killed by Krause.
-Haunted Castle happens in 1691.
-Shortly after Haunted Castle, Simon decides to avenge his father. For some reason leaves the whip behind and uses another whip. Gets killed by Medusa.
-Desmond takes up the mantle.
This scenario seems to work.
Would even explain how the heck did Desmond and his sisters track down the mansion -- by following Simon's trail. Scratch that. Forgot that this is info shared by Giovanni Gandolfi.
And I say SEEMS because something about that "20 years ago" thing is still not flying. Also, the lack of any mention about one more brother on this story really doesn't help the case.
maybe they were trying to leave in a story hook for a sequel that obviously never happened?
I'm not promising anything, but I might try to look into this.
One thing that strikes me as funny is how Dracula tells Desmond that "he worked together with Krause to bring him back to the world of the living." When Desmond questions him further, Dracula says that "maybe he has said too much."
Also, the epilogue says that a "looming presence remains over Desmond." I think it might be possible they pulled a Soleiyu Belmont here with Desmond.