So I had been under the impression for the longest time that Kostchtchie was drawn from Koschei (koss-chee) the Deathless, who has a kind of "lich phylactery" vibe going on:
Koschei's soul (or death) is hidden separate from his body inside a needle, which is in an egg, which is in a duck, which is in a hare, which is in an iron chest, which is buried under a green oak tree, which is on the island of Buyan (a mythical island that is the Slavic equivalent of Avalon) in the ocean. As long as his soul is safe, he cannot die. If the chest is dug up and opened, the hare will bolt away; if it is killed, the duck will emerge and try to fly off. Anyone possessing the egg has Koschei in their power. He begins to weaken, becomes sick, and immediately loses the use of his magic. If the egg is tossed about, he likewise is flung around against his will. If the egg or needle is broken (in some tales, this must be done by specifically breaking it against Koschei's forehead), Koschei will die.
Now, in Bestiary 4, the background for the demon lord Kostchtchie mentions that he was granted immortality after a part of his soul was hidden away in a physical object (a torc this time).
Also, Kostchtchie has the face of a rather old Slavic-looking man, as does Koschei the Deathless.
Kostchtchie is notably misogynistic, even gaining a favored enemy bonus against female humans and giants. In most of his tales, Koschei the Deathless is an outright rapist.
So I don't think it's possible to argue that Kostchtchie wasn't modeled to at least a small degree after the Slavic mythological figure.
Now, it's totally okay for the Pathfinder RPG to take a preexisting mythological or literary figure and change them up. (Asmodeus from Milton's Paradise Lost, for example).
But I cannot for the life of me think of why the pronunciation was changed in Bestiary 4 to make all the T sounds hard.
I don't speak any Slavic languages and can't read Cyrillic, but to my understanding there are certain letters in these alphabets that signify, basically, taking another letter and "doubling it up," sometimes with a very faint stop in the middle. That would be the "chtch" in Kostchtchie.
Now, the distinction in pronunciation between "Koschei" and the more correct "Koschtchei" would be pretty much undetectable to most English speakers, making Koschei or Koschey the natural spelling we would use if we were taking dictation and had never heard that word before (but maybe with a 't' between the 's' and the 'c' to make it clear it's not the Germanic 'sch' in 'Schrödinger' or the 'sch' in 'school').
So, what was the reason for the divergent pronunciation? Are there dialects of Slavic, or other sources of myth, where this is the preferred pronunciation? Was it a conscious choice by the designers? Or was it their best guess at a pronunciation? Or was it merely the pronunciation that gained currency among RPG designers (who are a very small crowd) at some point in the past and simply stuck?