Hi, I don't quite understand your message; when you buy a game on itch.io you don't get a Steam key.
As for the Linux version I decided not to publish it because the engine I use, Visual Novel Maker, no longer has support for this format and it is possible that it would give several errors. If you want I can give you the Linux version for free to check if it's playable.
I didn't know that you could play it on a Linux through Proton, if you finally play it this way, could you tell me what is your experience?
Thanks for your comments, best regards.