If you’re really going to release PyStan 3 ahead of the rest of Stan 3, then that’s a fine way to go and will render this discussion moot.
Did I not forward you the message from the NumFOCUS IP lawyer? I’ll do that now.
This is completely irrelevant. A copyright holder may release their IP under multiple licenses without conflict. For example, we did a GPL release of Stan for JStatSoft before they changed their mind about IP requirements and allowed BSD submissions.
According to the lawyer, that’s not required, as translated code is not considered a derived product. I think the reasoning here is that code is only copyrighted, whereas algorithms require patents.