hey Fabo!
There's a few questions that come up here so tell me if I don't understand what you're asking, okay?!
Yes, the power levers (throttles), the prop controls and the fuel cutoff controls are all mounted in the ceiling of the cockpit in the Twin Otter. They're from left to right, and it's really not as weird to have them there as you might guess. It's actually very comfortable and you forget about it after the first 5 minutes or so.
The reason the old girl is so slow is the DRAG... it's built with fat wings, fat struts, fat tires, fat everything. It slices through the air with about the same sleekness as your refrigerator would, if you pushed it off a 10-story building! But, it's built for stability at slow speeds, so slowing down in it is a JOY compared to most airplanes. Because it's built for that.
The power levers move forward for max power, and aft for minimum power setting. To use reverse, you have to twist the power lever handles themselves; they rotate slightly and that allows the main levers to continue aft, to go into beta and reverse. So, after landing, you'd twist them and also bring them back aft to slowly go into reverse -- slowly at first, anyway, because if you are fast on the controls, and one goes into reverse and one doesn't, well, you have suddenly become your own entry into the First Annual Runway Rodeo Competition, and you're headed for the weeds off the side of the runway.
Does that make any sense??