At some point you might want to consider whether or not your oil leak is in fact the oil from the reduction gear box rather than engine oil.
It is correct to assume that overfilling the engine oil would create a situation where oil would vent from the crankcase breather. On many models of scooters that oil would collect in the bottom of the air box. Depending on the construction of the air box, the oil then might seep out a seam making a mess. Other common side effects of too much oil in the air box is a poorly running engine as the engine oil could get sucked into the motor which in turn could foul the spark plug and / or create smokey exhaust.
However, you had two very big problems with your final drive. First when the bearing in the swing arm failed, all the pre-loading of the output shaft went to zero. Normally the output shaft bearings are preloaded by torquing the wheel nut to approximately 90 foot pounds?
The second problem is more subtle but much more heinous. The output shaft is normally supported by three bearings. When the swing arm failed it put the output shaft into a cantilevered configuration. Any force generated at the wheel, such as application of the rear brake, bumps in the road, even converting the torque from the engine to either accelerate, decelerate, or hold a steady speed would cause asymmetric loading of the remaining two bearing. Running the output shaft in this configuration would generate a tremendous amount of heat which could cause additional failures, such as a leaks from either the output shaft oil seal, and / or the input shaft oil seal. Complete bearing failures are often preceded by the failure of the bearing cage. This would effectively contaminate the final drive oil with bits of metal that could also cut the oil seals as well. A leak from the input shaft seal would allow the final drive oil to leak into the case with with the CVT belt.
Something to consider anyway. Chapter 9 of this service manual provides a lot of good information regarding the final drive.