How can a person suffering from urinary incontinence combine two prayers with a single ablution, and how is the final time for each prayer determined? And is it obligatory to repeat prayers that were performed based on an incorrect personal estimation of the prayer time?

It is permissible for a person afflicted with urinary incontinence to combine the Dhuhr and Asr prayers, and the Maghrib and Isha prayers, either by true (real) combination or by apparent (illusory) combination, with a single ablution.

True combination: Performing both prayers within the time of one of them (either by advancing or delaying).

Apparent combination: Delaying the first prayer until its very last moment, such that the time for the second prayer immediately follows.

What you did, performing Isha 22 minutes after Maghrib, is not considered a true nor an apparent combination, and you must repeat the Isha prayer with a new state of purity (tahara).