What are the Buddhist norms & guidelines for rebuilding a relationship after a breach of trust?
I can't think of any instances where the Buddha specifically addresses your question. However, you may find the Patience chapter in Santideva's Bodhicaravatara (Way of the Bodhisattva) quite helpful. Of course, the whole chapter is profoundly useful (as is the entire text), but you may want to have a look, starting at verse 38 of chapter six. Here's a link: https://www.tibethouse.jp/about/buddhism/text/pdfs/Bodhisattvas_way_English.pdf
In a non-Buddhist relationship (such as described in AN 4.53), either one or both parties don't follow the five precepts and other Buddhist principles (such as the principles listed in DN 31). Obviously, the non-Buddhist relationship is a "doomed" relationship, where, per AN 7.63, one party is the killer of the other party or the killer of the relationship.
Where as in a Buddhist relationship, since each party has the intention to follow the five precepts and other Buddhist principles, any transgressions are willingly reconciled using honesty (sacca), training (dama) & patience (forgiveness), per SN 10.12.
Also, Buddhists make confession to the Triple Gem, as found here at the top of page 21, middle of page 23 and bottom of page 25.