Doable. Assuming no delays coming in, you'll be able to take a train to the Hauptbahnhof around 11 and you'll have to take one back to the airport around 1:30. That leaves you with two hours strolling through the city centre and the lake and grabbing some street food. As for your other questions, a) nothing really interesting to do around the airport and b) Swiss immigration is super-efficient (especially when compared with the US).