On closer inspection, I indeed put the zeroth level of the potential energy at the starting point, I just didn't really realise it untill you pointed it out. When putting it at the center of the loop, I als got ##\frac{2}{3}R##, but I know this is just the same answer as ##\frac{-1}{3} R##...
I think the normal force becomes zero when the toy loses contact. But when I use this and calculate the angle you specified, and then calculate the height h coresponding to this angle, I get the non sensical anwser of h = -1 / 3 R, which seems really strange. Maybe I made a mistake that I didn't...