Can I give away a steam game and promote a non-steam game (google play) on my giveaway description?

Sure, people do this all the time, but I think it will be more productive to giveaway also your playstore game at the same time, people will look forward even further for the games

Yes.

As long as your giveaway is not fake, yes.

Unfortunately, it looks like I cannot purchase and store the steam game in inventory for gifting. It looks like I might not be able to giveaway because of this so I will close this discussion.