Is it really such a bad thing for your launch/event to crash a platform? Nobody is going to decide not to buy Silksong after all because it's so wildly popular it brought down Steam. It generates a great deal of positive headlines. To me it seems like a good problem to have.
2. The game has no DRM and Steam preorders (in my experience) download the game files so people can play instantly on launch day. (They call it 'preloading'). For a game as highly anticipated as this, it'd likely just be cracked, leaked and pirated the moment preorders came live.
Aside, your second point is incorrect. The SteamDB folks have a public write up on analysis of the preload system: https://steamdb.info/blog/steam-download-system/