Lowest full on rpm/speed?

Not sure how to ask so hopefully this will make sense. I know (or think so anyway) when you first open the throttle it's something like slipping the clutch on a non-CVT vehicle. At what point in either speed or rpm is it no longer slipping but fully engaged?

I presume it is better for the drivetrain to be fully engaged and not slipping? I presume the same goes for fuel mpg?

The Swing has a real clutch, in the rear driven assembly. It is a centrifugal clutch with 5 friction pads or shoes.  The clutch slips until the drive belt turns the clutch arms and shoes with enough centrifugal force to lock the whole clutch/driven assembly together. Springs are built into the clutch to disengage it when the rpm drops and to allow the clutch to lock up gently. Clutch slip depends on the rpm of the belt, the weight of the bike and its passengers, road and wind resistance, etc. With very light throttle and light load, the clutch locks at about 20 kph. With large throttle input and heavy load, the clutch may not lock until 50 - 80 kph. If the clutch is glazed or worn, it may not lock at all, and when you accelerate, the engine speeds up but the bike doesn't.

With lots of rubber band/cvt drive scooter miles----I think SilverWing works great in the driveline dept.....as the engine is very flexible---mine works good from lockup to at least 150kph.

Fuel mileage....at least one part, depends on how heavy you are with throttle hand.