Dear Dan, you ask too damn many questions.
But I'll take a shot at some of them. The 140 watts I think refers to how much power the amp consumes. The 400 watts is how much power another device plugged into the two pronged extention outlet can use. Up to 400 watts, in other words.
I am pretty sure that your amp wants to see a four ohm load. Two 8 ohm cabs plugged into the amp would yield 4 ohms. Check the ohmage on your cab. I bet it's eight. Running only one cabinet is no problem. It means slightly less volume, but it's barely enough to notice. I am pretty sure you can check the speaker ohmage with a ohm meter, but you'll have to get that info from someone else.
Does your head have reverb and tremelo? If so, there's your answer to what the footswitch is for. If you have only one or the other, one of the switches is probably just there to fill the hole. Maybe they only manufactured one version of the footswitch. Plug it in and see what it does.