The answers:
1) If the ABC company does not give you NOC, there is nothing you can do about it. There is no system of appeal.
2) I do not have any idea on this but if your company stamps an exit visa on your passport, you must leave withing seven days or face a penalty for each extra day of stay.
3) Yes, there is way to get your End of Service benefits if your company refuses to pay. Just lodge a complaint with the Labour Office. The procedure is very simple and the Labour Office will force the company to pay. Their action is also very fast.
4) If you do not get NOC from your current employers, you will need to stay for two years out of Qatar before you will be able to return on a new visa. This is as per current rules.