Reminds me of my take on this https://www.furaffinity.net/view/53474437/
But she is two legs merged together.
How does she work up below the dress? Does she have two thighs merging into one shin and large foot.

Yeah I remember that series you did - lovely work :)
This drawing was a little favour I did for a friend. Her leg is supposed to grow out at the middle of her hip (no idea what that will mean for her "private parts"...) sporting a massive tigh and shin.
I hope this will answer your question - I really did not think everything through when drawing her ^^'

it does. And it has issues 😺
Having two legs merging like on my recent mermaids and the one legged demones, for her its obviously two legs merged, for the mermaids like catfish I not thought about it much except it end in an fin with front and back fins and the bottom is an foot for jumping, a lot like seals but better designed.
For classical mermaids where spine is the tail both openings has to be in front.
Plan to continue the one legged demones soon, after wrapping up the mine.