Q1 It is only partly true. For non manual workers not more than RM1500 but for manual workers, they are covered even though their salary is over 1500. Better read the First Schedule of the Employment Act more closely at http://pesaraonline.net/1stschedule.htm
Q2. You got it wrong. There is not limit in salary range. IR Department deals with dismissal cases among other matters. Read the Industrial Relations Act. The Act never mentions anything about salary.
Q3 For this type of workers, they can go to court to make claims against their employers.
Q4. Both the Labour Department and the Industrial Relations Department can help them depending on the types of problems.
For your information, labour laws are pretty complex.. Learn them slowly, read more or attend some courses, seminars and so on.