Does a revocable divorce (ṭalāq rajʿī) become an irrevocable divorce (ṭalāq bā’in) merely by the husband agreeing with the wife to waive his right to take her back during her waiting period (ʿiddah), and is the husband's taking her back considered valid in this case?

Revocable divorce does not become irrevocable by agreement of the spouses, for the right of return (ruj'a) belongs to Allah Almighty, and no one has the right to relinquish it, because relinquishing it is considered a change to what Allah has legislated. So, as long as you have divorced your wife with a revocable divorce, you have the right to take her back during her 'iddah (waiting period), whether she consents or not. Any agreement to relinquish the right of return is invalid. If a dispute arises, it should be resolved through the judiciary.