1) depends on an individual basis I guess. my dad's got some on his 1980's Mercian, which has really wide bars (44s I guess). He's a Clydesdale, so he added them for posture purposes. he's got plenty of room to mount an average sized light toward the stem clamp. as long as you don't have too much stuff in the cockpit, you'll probably be ok.

2) These levers work by pushing on the cable housing. You need to make 1 cut in the housing for each brake, then drop the lever in the cut (add some little metal fittings if you've got them, though they'll work adequately without). The cable threads back through the whole thing like before, except now you've got an extra brake lever.