Stair Rails Wall Side
Most people are right handed, but "most" of humans are versatile enough to use their other extremities.

But your conundrum is really going to satisfied best if you go middle ground and trying to satisfy most people who'd have an issue with "switching hands" I'd say.

So install the railing consistently, all on one side... ( Right or left, depends if your coming or going ) 

But rails on both sides throughout traversing stairs takes care of it all...Can you do that ?

Move the light switch either up or down. Probably down, because most people would "complain" that they had to "reach too high" to operate it...