If you can't get an earthed 3 prong snub-nose adapter, you can always buy a cheap chinese 3 prong ~1.5m lead and then just trim the lead and put your own 3 prong connector on. That's how I made a short lead for the apple PSU.
As above, it's normal, but it's annoying. The tingling, even stinging feeling seems to punch well above its weight for what the 100-250uA RMS would seem to imply. When I made my thread I only measured current with a multimeter, I didn't examine the waveform.
Also worth noting, of the macbooks Ive tested, the chassis isn't conductive. Despite looking metallic, it's powder coated with an insulator, so the current is capacitively coupled.
To that end, the surface area of what is touching the chassis will influence the current you feel. If you directly touch the chassis with a probe lead, the voltage and current will be less compared to placing a metal plate on the chassis then touching your probe to that.