porting matlab code on to TI OMAP L138
1 - Too broad, please be more specific
2 - Most functions are supported, you can check the simulink blocks to see if they support code generation. That being said, even if a function is supported, it does not mean that it makes sense to have it in your code. You can also convert Matlab functions to C/C++ but there are restrictions.
3 - What you describe is a model-based design where your simulation model and your executable are based on the same source. I use model-based designs in power systems, I use a controller for simulations and I use the same controller for code generation.
It works well, reduces development time, helps eliminating bugs. However, it's not a silver bullet, it's not as easy as Mathworks wants you to think. You still need some coding skills.