Im making a game where you manipulate a skateboard. When I rotate the skateboard sometimes some objects are not behind different objects. How to properly display my skateboard?

Hi im thinking about making a skateboarding game where  you control yout skateboard like a fingerboard. It would be similiar to touchgrind on iPhone, but I want to make it a little bit different. I have made already a prototype using Unity to pass my laboratories at my technical university. Now i want to make money with that game. I dont have 3k$ to buy unity pro and i would really like to work in ADT. I was searching for a library to automate low level openglES things i dont want to care about. I found rajawali library but couldnt get it working. Now i found that and im pretty happy because it was the thing i was searching for.
Ive done so far:
*Edited that HelloWorld app
*Made a ugly model of a skateboard to start coding.
*Imported the deck and wheels(wheels spin! )
*Made a class for skateboard physics.
This is how it looks so far

I would like to post my progress and some questions.
First question:
In Unity every transformation was multiplied by time elapsed beetween every frame. Is it a good way to make the game speed equal even at lower frame rates?

If your having problems understanding me im sorry, English isnt my first language.