Is a young man who has proposed to a girl considered her husband after a betrothal ceremony has taken place and two witnesses have been present? And is it obligatory for a girl who has separated from her husband for more than three years to observe the 'iddah when a divorce occurs?

Divorce does not occur merely by the wife's separation or by filing a divorce lawsuit; the wife remains under her husband's marital bond. Therefore, your acceptance of a suitor and performing a marriage contract with him is void, because it is a contract with a married woman. It is not lawful for you to marry until a final divorce decree is issued by the court and the waiting period (iddah (definition)) has ended, if consummation occurred. Your separation from your husband does not negate the iddah. You must immediately cut off ties with this young man and repent to Allah.