Is the former wife entitled to request alimony again after previously waiving it, especially given the change in the former husband's financial circumstances for the worse?

If the divorce is revocable or if the woman is pregnant, she is entitled to maintenance only for the duration of the 'iddah (waiting period). She does not have the right to claim it after waiving it, for "what has been relinquished does not return." As for the children's maintenance, she does not have the right to waive it, because it is obligatory for them. However, if she waived her right to claim from you what she previously spent on them, then her right has been relinquished. And if she spent on them without the intention of seeking reimbursement from you, then that is a gift from her. She has the right to claim maintenance for the children from you in the future. There is no blame upon you if your eldest son takes over the spending on your behalf, especially given your straitened circumstances; rather, it is obligatory for him to spend on you when you are in need.