~Unwanted Fodder~
Nothing is mentioned on Shinras mother in great detail and that is something i personally always found strange given how loose tongued Shingen can be but my belief is that it's possible that...Celty is Shinra Kishitani mother...
In all honesty even Shinra name is questionable [alledgely]...in egjytion it means ''great evil'' while in japenese it means ''Black/Dark Silk'' which is stange name within itself to call your own child. Shingen has acknowledged Celty as evil hasen't he? Could the dark/black silk be in reference too Celty Sturlusons smoke that she can manipulate? So in a result he is naming his child after the mother ?
The timline dosen't give much sense of closure between Shinras birth and the undefined time his father had saika and learned of dullahans but even if it is before or after he learned or obtained Saika it dosen't matter because they both could be 'involved' before he knew or because he did...This could also explain why he dosen't ever show his face around her though it's said that shinra and shingen look identical but this can't be easilly be proven.
As a result as Shinra aged that is why she accepts him romanticially because he looks exactly like Shingen back when they fell in love despite this it could also explain her natural attack instinct to his romantic advances despite accepting him as a partner because part of her vaguely knows so it's possible her feelings for him is confused due to her lack of head.
It's even possible the Shinra knows she's his mother hence his urgancy to not let her find her head and why he cut it with his own Saika so she would forget
Dullahans can't lie so it may be good to recall what she has said overtime because her intention was to leave despite loving Shinra so dose this prove my point further that she was leaving out of pure disgust with herself? what mother couldn't consider herself a monster when they possibly slept with their child [them doing such has been hinted at and thinking of Shinras nature she may have been violaited during sleep considering his peeping tom tendensies]
It is said that Shingen dosen't even approve the love between his son and Celty which is weird that he will fully oppose because of his scientific curiousity since i am sure he will be fasinaited if Celty could even produce offspring but thats a double edge sword in my point; since he is not sure she can breed is why he is not approving it because every father usually want their genes to continue...
In short if this is true Shinras' nature is worse then i ever anticipaited...