This one is very nice!
Not really important, but with radius, lets say 2.3, the black curve crosses the blue one when descending. Actually it must follow the blue one again.
The bar is fine. Would it not be nice when the bar was as long as the height of the object is starting? Then it represents the "h" in E=m.g.h
The green bar is rounded at the top. Could it be made straight? Looks better to me.

The last request was: let stay the object in its curve with not changeable radius. It must stick to the curve. Now he will make it to go round, ore not.
It depends of the energy he has when he is going into the looping/curve
The first image shows what you have been accomplished now, WOW!
The second one is the second request. You -or a marble- has to stick to the curve and gets round or not...
I hope my explanation makes you understand what I mean.