E-scooters in Europe are generally soft-limited to 25kph (15.5 mph). This is seems like quite a good practical measure, although it's "problematic" from a software freedom perspective. It's quite hazardous for the rider to go much faster than that anyway, scooters are much less stable than bikes.

Actually there should be speed limits for shared paths even without ebikes. Here (Hungary, EU), I think it's 20kph (12.5mph). Even that is too fast to hit someone, but of course, the point is that you'll have more time to slow down and react.

I'd say they are somewhat effective. I don't ride much on shared paths, but when I do, I usually see people not going full speed. After all, unlike with a car, it's also dangerous for the biker. The only concern might be that you may experience speed differently on an ebike/pedelec since you have to put in less effort. I've never tried those so I have no idea.