All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, is His slave and Messenger.
Your question included several matters, the answer to which can be summarized in the following points:
First: It is impermissible for an employer to delay the payment of the salaries beyond their due time. The Prophet said: “Give a laborer his wages before his perspiration dries up.” [Ibn Maajah, Al-Albaani: authentic due to the collective versions narrated through different chains of narration]
Second: what this employer did, withholding the employee’s salary or some of his rights and denying them because there is no written and documented contract between them, is considered a grave misdeed. Abu Hurayrah narrated that the Prophet said: “Allah, The Exalted, Said: ‘I will be an opponent to three types of people on the Day of Resurrection: … (one of which is:) a man who employs a laborer and gets the full work done by him but does not pay him his wages.” [Al-Bukhari]
Third: there is nothing wrong with dealing with some of your former employer’s clients, and there is no sin on you because of it. It is not incumbent on you to refrain from dealing with them and offer them the lawful services they requested that are not related to your previous job. Moreover, there is no sin on you for benefiting from the experience that you gained from your previous job. What is impermissible is for an employee to disclose work secrets and the like.
Accordingly, you bear no sin – Allah Willing – for dealing with those clients or others who request lawful services from you, even if they are your former employer’s clients.
Allah Knows best.