It's made by XY Powersports or Xingyang, I can locate manuals and wiring diagrams for the XY1100GK. Just nothing on the XY600GKE is all.
The issue with removing the speed sensor (which when tested, passes), allows the machine to operating as intended makes it even odder. Could there be a child safety, seatbelt safety, or 4x4 L that prevents the machine from moving faster than 15 mph when switched? I mean the manual you provided has the exact engine, but the wiring is slightly different.