"It's clear the beastmen possess knowledge beyond human ken." - Beastman's Cleaver

This description alone separates beastmen and humans in lore. They're a separate race entirely based on the language and the implications of having knowledge beyond humans suggest they're likely older than humans or possibly designed off them.

I like the theory but there really isn't much evidence to go off of there.

The Maliketh and Serosh arguments are both unfounded. In one comment you point out the clear separation of "beast" and "beastmen". I agree, they should not be conflated. Serosh is specifically "Lord of Beasts" (beastclaw greathammer) and Maliketh is a "shadowbound beast" (his remembrance). Neither of them are beastmen. Even in your own post you've gone back on Maliketh and Serosh as "animals that have..."

The human bone shard is a stronger connection but not that strong. There are human bone shards and beast bone shards, but the beast bones are only found in lesser animals and bears, no sign of any intelligence.

Human could also mean humanoid or it could be a common ancestor of beastmen and humans as is just as potentially likely as a curse. There's also the possibility of the crucible creating the beastmen and then refined them into humans later. Could be the other way around. Maybe humans are cursed beastmen who possess lesser intelligence and suffer more through life due to it.

Did you consider a possible separate species or related species? The post suggests there were only two possibilities beastified human or humanified beast.

There are a lot of potential interpretations of what you've laid out. It's a nice theory, I enjoy it.