I do not understand why people don't put full information at once when they put a post...
If you have not received your salary, you are under no obligation to complete even your notice period as the employer has already breached the contract of employment. If you are completing your notice period, then it does not matter if the haven't found anyone to take over from you. That is not your problem.
Though I am wondering why you are bothering to stay when they havent paid and I don't think they will pay if they have a financial crisis. They would prefer to pay existing staff not those who left.
The actual problem is with the relieving letter. You need to focus on what you need to do to get it. Forget about legal action or legal rights in these cases. Instead try to be diplomatic and get it. if you get a job, you need to tell the employer to wait till you finish your remaining notice period. Any employer who insists on immediate joining, is (in my books), a bad employer.