Video Stutter With Multiple Logged-In Users
I experienced a similar problem with a 2012 MacBook Air with two users logged in. Both users logged in, stuttering video via AirPlay, OS X 10.8, 8GB RAM, plenty of room on the SSD. When using an iPad or iPhone on the same wireless network and AirPlay, no stuttering.

Logged out the second account, no stuttering.

I haven't solved any of your other issues, but did confirm the concurrent login problem.