Call main.lua script at each game loop iteration - is it good or bad design? How does it affect on the performance (relatively)?
Maintain game state fro
I don't like C++. But I do like games.
My approach might be a bit atypical: I do everything I can in Lua, and only the absolute minimum in C++. The game loop, the entities, etc are all done in Lua. I even have a QuadTree implementation done in Lua. C++ handles graphical and filesystem stuff, as well as interfacing with external libraries.
This is not a machine-based decision, but a programmer-based one; I output code much faster in Lua than In C++. So I spend my programmer cycles on new features rather than on saving computer cycles. My target machines (any laptop from the last 3 years) are able to cope with this amount of Lua very easily.
Lua is surprisingly low-footprint (take a look to luaJIT if you don't know it).
This said, if I ever find a bottleneck (I haven't yet) I'll profile the game in order to find the slow part, and I'll translate that part to C++ ... only if I can't find a way around it using Lua.