So this question was asked years ago and I'm sure @Archivist has come up with his own solutions long since.
I found it intriguing on stumbling across it yesterday and have had some thoughts subsequently.
I would reserve Alchemy and Witchcraft for Uropan characters and allow those characters access depending on their background as follows:
Those from a lower class, less educated and/or from a rural background, would be able to have Witchcraft.
Those from a higher class, more educated and/or from an urban background, would be able to have Alchemy.
For those characters that want to stray from the above guidelines, they need to have a backstory with plot hooks to justify it!