I do not think there's an automatic way written yet to make skype disconnect when you go off WLAN. If you expect to be out of WiFi often enough, I would recommend keeping Skype accounts that you've entered disabled while using the device, and only turning it on once you get onto your WiFi network.

I think there should be a way to write a script that automatically enables or disables Skype, when you get on WiFi or get off of it, but I just don't know what that script would be.

I don't know about a digital clock on your homescreen, but doesn't the one in the top left count as a clock already? Or do you need one that's bigger and/or noticeable? I'm sure there must be one out there, I just don't know of any myself. Look around in the Download section of the App Manager. If you still can't find one I'm sure you could post a request thread and someone will make one for you in their spare time.