Anyhow, Haeckle's theory of the Biogenetic Law is largely debunked.
Well, yes and no. While it's true that species don't recapitulate their entire evolution fetally - an idea stemming from the flawed notion that evolution was a straight-line process from simple organisms to more complex ones - it is now known that Haeckel was looking in the right place, at least. It's known now that the process of evolution of morphology happens primarily in the form of changes to the genetic program of a species' fetal development.
I mean, obviously - making changes to the blueprint of a building before the building is built results in changes to the building. But after the fact, drawing in an extra wall or bathroom onto the blueprint of a building long since finished produces nothing at all.
That said there are many cases where developing embryos do seem to recapitulate aspects of their phylogeny. Whale embryos, for instance, develop and then lose hair and legs. It's not clear why this should be the case unless whales inherit, evolutionarily, the bulk of the fetal development program shared by other mammals.
Re: How Creationism Explains Human Tails (It Doesn't)
I'm sure you've heard it a million times already, but the evidence for common ancestry could logically be used as evidence for common design.
No, it can't. Common design could explain why both land mammals and cetaceans nurse their young from teats and are warm-blooded. But common design can't explain why fetal cetaceans grow the same pelvis and hair land mammals do. (Later in development the fetus loses the hair but keeps the pelvis.)
Dean Kamen is the designer both of the Segway scooter and the kidney dialysis machine, but the Segway doesn't have any parts from dialysis machines. Common design explains the commonality of useful adaptations in multiple applications, but it doesn't explain why you find commonalities even where they're totally useless, like the whale's pelvis. Common ancestry, on the other hand, explains both.