Will your game get rejected by Apple for writing your iPhone app in C++?
Of course not.
Many of my friends write their game logic in C++.
And Unity3D developers can write their games in c#
By definition Apple does allow Objective C, C and C++. All other languages are possible but questionable.
cocos2d-iphone uses Box2D, which is written in c++.
I wrote my game in C++ and using cocos2d-x. It got approved by Apple recently.
You will only get rejected if you write bad or not performing well code. And of course, you are subjected to your reviewer. But no, you will not get rejected for writing a game with predominant c*+ code.
No way you will get rejected for releasing c*+ game. I just released my game in cocos2dx. My game even uses html, javascript and php for interface, so it means you can mix a lot of coding in just one app, just as long as they dont break. 
I have several all C++ applications on Apple Store. Cocos2d-x applications pass acceptance tests for iOS and Mac Store without trouble.
App will be rejected if i download LUA scripts?
App will be simple client which only downloads LUA scripts.