There are a few different issues interacting here.
The “family mode” users that require PIN are a child protection measure, and are not connected to Family Sharing. Remove the PIN from all adult accounts. Now you will see your whole library and be able to go to the store, and when you switch to your son’s user, he will not be able to go to the store and will only see the games you have done “Add to Family Games” on. This is how my library is set up: sharing to my partner and child, only child’s account has PIN.
I don’t know the cause of your experience with the keyboard, but if you remove the PIN from your own account, that should make it less painful.
This is just the way the Steam client works, not a Deck-specific feature: you are logged into one account until you change it. The PS5 is the same way.
In my experience, failure to separate game state between users is a game-by-game problem. Most Windows-native games running in Proton separate their saves by user correctly. (I do not know whether this happens because the Deck generates a completely clean Proton environment for each Steam user, or whether the Proton environment is shared and the game is just doing what it would do on a Windows PC to separate saves.) The games where I have seen saves wrongly shared, ironically, are all games with native Linux ports.
If you haven’t already, switch to your son’s account, unlock the PIN, and go through all the Steam multiplayer/chat settings. We have all that turned off for our child. As far as I know, a game family-shared to a user should behave exactly as if the user owned the game, from a functional point of view.