Open the Properties window (right click the game in your Library->Properties), navigate to theGeneraltab and un-tick theEnable the Steam Overlay while in-gamecheckbox. You should also un-tick theKeep games saves in the Steam Cloud.
You can search for nearby Switch players underSearch for Local Userson theAdd Friendsmenu of the user page. To find one another, you each have to click the same symbol. Our best guess is that this is a parental control feature, albeit not an incredibly sophisticated one. Image used with ...