I spoke with Squenix' support as well so they could lodge the frustration re: the steam client, and asked why it wasn't possible to change licenses or remove a license so I could rebuy the Windows one. They've said it's a restriction Steam has imposed, and I could ask Steam support about it.

In the meantime, I noticed that you can remove a key from your Steam account entirely (delete it); has anyone asked Squenix if we can prove that the key is no longer on our Steam accounts if they are then free to purge the Steam license from our service accounts? I doubt it would automatically fall away, that seems too simple, but maybe with intervention? :/