Change just the o-ring first, it may help.
Gear does not matter.
The bearing ball is part of the switch, it does not fall out.
Some oil will leak, but not much, it is not like oil pan drain.
Just go slow and gently and make sure the switch does not pinch the oring.
Size of the oring - it is 18 x something. Measure the one you have and try to find slightly bigger.
Or the switch itself could be leaking, you will see after new oring.