What is the ruling on a woman who vowed not to eat chocolate and cakes, and obligated herself to follow a water-only diet without specifying a duration, and now wishes to retract these vows? Is a single expiation required for repeated vows? Is it permissible for her to resume eating and stop the diet after expiation? Can she resume the diet later without a vow?

If you doubt whether the covenant was made, then nothing is binding on you, and the default is that it was not made. If you are certain that you covenanted with Allah to abandon something, then this falls under the ruling of an oath, and your intention specifies and restricts the oath. So, if you intended to stop eating something for a specific period and did not intend it perpetually, you do not break your oath by returning to eating after the intended period has ended. If you break your covenant, having covenanted with Allah to abandon it, then you must offer expiation for a broken oath (kaffarat yamin). After the expiation, you may do as you wish. It is advisable not to make too many covenants to avoid hardship. You can abstain from a certain food for a benefit without making a covenant or an oath.