You don't adjust the air flow meter, the meter's job is to give accurate air flow readings to the engine computer to it can adjust the fuel-air mixture. If the air flow meter isn't delivering accurate information to the computer then it needs to be cleaned/repaired, not adjusted. Most allow no type of adjustment anyway. It may be it just needs a clean, how ...
Usually very little if the two pipes are on the same silencer box.
If, however, there are two rear boxes, one on each side then the system tends to be less restrictive.
Two pipes are usually for a "sport" badge... etc along with the "go faster" stripes...
A larger single can be very close in effective diameter to two singles depending on the sizes.
Removing the gearknob on a Vectra C is literally a case of pull upwards. Some are really stiff, some are really loose. My Signum (based on the same model) is so loose it likes to slide off when the weather gets hot
Be careful when you do as there is a spring that sits between the lifter and the gearknob.