Servo assignment is flexible, you can assign the servos as you wish. As to your other questions I can advise as follows
Mixer, Flaps SW--- this should be a switch you wish to use to activate the flap control, if you want the flap control to always be active then leave this blank.
Set Mixer, - use the Off (offset) to change the neutral position of the flaps, then the flap control input will apply from that point.
Note you need to make sure that you have assigned H1 to work in the correct direction or any other mix that uses the Flap control input will work in the wrong sense, e.g. Elevator compensation in the Elevator+ mixer.
