If I'm not at home I would like the MBP to lock the screen after 15 mins, but not while I'm home. So if my home-wifi isn't detected I would like the "lock screen" to be enabled. Is this possible?
Yes, when the Caffeine icon is clicked on the Menu Bar, it overrides screen saver and sleep settings. Click it again to turn it off. You can still manually activate your screen saver with Caffeine activated. Screen savers are based on time, not the absence or presence of a particular WiFi network.