Is it considered valid for a wife to return [to her husband] after being divorced on the condition that she waives her rights, and is there conditional divorce in Sharia?

The matter does not go beyond two scenarios:

1. The wife's waiver of maintenance in exchange for divorce by prior agreement: This is considered Khulʿ (divorce at the wife's instance), and by it the wife becomes irrevocably divorced with a minor irrevocability. Her return is not permissible except with a new contract. 2. Her waiver of maintenance after the divorce has occurred: This is a divorce and not Khulʿ. As long as it is the second divorce, you have the right to take her back without a new contract and without her consent, as long as the waiting period (ʿIddah) has not expired.

It is not the right of the wife or her family to prevent the husband from marrying a second wife or to request divorce for that reason.