Dimmer under potential but no load
For Q1: (a/b) are both sort of correct. A dimmer should never be a variable resistor as that would consume a large amount of power and would cause fires or fail easily. Dimmers are made from SCR's which are active devices which save power and don't heat up nearly as much. So long as the dimmer is rated to handle both lights, then it shouldn't dim when the second is turned on.

"Modern dimmers are built from silicon-controlled rectifiers (SCR) instead of variable resistors, because they have higher efficiency."
http://en.wikipedia.org/wiki/Dimmer

On the other hand, the rest of the circuit (wires) will have to carry more current when the second light turns on. If the wires aren't large enough they'll heat up causing a slight increase in resistance and a corresponding slightly lower amount of current to reach each device.

For Q2: (a) is correct. The dimmer will be wired in series with the lights/switches so lightning would still have to travel through the open (off) switches to damage it which is the exact same even if the dimmer is wired after the switches.

You look golden to me.