i have heard several versions on this one. i drop my friend back home. that guy is around 70 kilos when i myself am 57 kilos. we go in an activa. am not sure if this will cause a drop in mileage. if it does,then how much?
going doubles in two wheeler: will it reduce mileage?
the milage claimed by the company will always be on test conditions ( like standard load of 150 KG on the vehical or something like that). single riding always has better milage than doubles.
just like doubles in bicycle consumes more energy 
Yes.
Yes. Also, if you read your manual, you will see that the recommended tyre pressure for the back tyre is more if you have a pillion than if you don't. So I recommend that you fill the higher pressure (with pillion). Irregular pressure checks does impact mileage significantly. Check at least once a month.
of course.... but then... it doesnt increase more than about 5-10% of the mileage, negligble.