I'm pretty sure as long as they are on my HDD, i can play them with an unconnected steam account. That's how we shared games 15 years ago, instead of trying to find keys or cracks, the person owning the game installed it on an external HDD, and with a few weird manipulations that was enough to play it with a disconnected steam account. Maybe that changed?
Anything that uses the steamworks SDK is going to require an active account to initialize properly. That may or may not be a fatal error, it's up to the devs to decide.
It varies wildly from game to game. It's very possible to use steam as simply a means to copy the game files onto your PC, or integrate with it so tightly it's 100% required to have an account in good standing (as well as one with the game publisher, or whatever), or anywhere in between (a lot of games make the integrations optional, which is the best solution IMO).
