Your employer has all the obligations to sent you home and cancel your working visa before it will expires, BUT YOU HAVE TO REMIND THEM TOO, to avoid any problem.
Regarding your 2nd question; I think you can enter with different kind of visa even without NOC but you will find it difficult to be issued another working visa if you are eyeing for another position in different company.
Your employer has all the obligations to sent you home and cancel your working visa before it will expires, BUT YOU HAVE TO REMIND THEM TOO, to avoid any problem.
Regarding your 2nd question; I think you can enter with different kind of visa even without NOC but you will find it difficult to be issued another working visa if you are eyeing for another position in different company.
Good Luck.