diademhill, I am of the opinion that the back end of a cut fish could never heal in such a way that the dorsal and anal fins could meet completely at the base, much less that 8 fish could heal in
exactly the same way, but you're welcome to your opinion. Remember, though, that this condition can be either genetic or developmental, and if you're only seeing one fish in a spawn like this, that speaks of it being developmental. Random mutations can pop up of course, but if you've seen more than one tailless con in your lifetime, from separate large spawns, it's probably developmental... mutation rates are, as a rule,
incredibly small.
Her bum looks that way because the back part of her spine is missing, basically... muscles associated with the caudal peduncle are also gone, but swimming muscles associated with the rest of her body are still present. It makes a little dimple-like area at the back because the muscles and bone they normally connect to aren't present so they just converge and apparently connect at the base of her spine. To me, that's actually further proof it's genetic... entire muscle groups are missing instead of just pieces being gone, and the shape of her body is altered to fill in the "gap" where the missing parts would have been.
Also, of course she'd be from a batch even if it were genetic... they BREED for this trait, remember?
I read long ago somewhere that it is a dominant trait, so even outcrossing a sweetheart parrot to another normal fish would produce tailless fry (assuming, of course, that that information is true.)
The problem with snipping the tail as early as you describe is that fry that size are much more fragile... glass fish, though small, are adult or nearly adult when dyed. It's kinda like how 3-4 week old betta fry can die at the slightest disturbance, but once they hit two months you can neglect them a good long while before they complain. The issue is really the developmental stage of the fish, not the size.
EDIT: Geez, sorry for the novel, haha! I just have a lot of points I wanted to make
It's just, like I said, this is the only batch of fish I've ever seen that looked anything even close to genetically tailless, aside from one I spotted on Aquabid a few days ago... same rear end as my girl.