Yes 1 & 2 join together = then it becomes overflow to the road surface. If the bike falls on its side, or accident , the carb should dump its fuel right out of those pipes 1 & 2
#3 = Yes join both bowls there , the accelerator pump actually sends fuel to both bowls , to squirt fuel towards the cylinders.
Yes it is needed to stay there because both cylinders actually move fuel back and forth , and sometimes it is to help escape some air pressure on to the road if it builds up in the float bowls.