I’m not sure "Can trust ever be rebuilt to 100%?" is even the right question.
I think the deeper issue is safety.
Infidelity does more than prove that someone was dishonest. It proves that, at least during that period, the unfaithful partner was willing to make choices that exposed the betrayed partner to harm while withholding the information they would have needed to protect themselves.
So maybe the real question is, "Can I ever feel safe being vulnerable to you again?"
Safety includes trust, but it is bigger than trust. It is believing that your partner will consider your wellbeing when you are not there, protect the relationship when nobody is watching, tell you things you have a right to know even when disclosure is costly to them, and not secretly make decisions that radically affect your emotional, sexual, financial, or relational life.
That is also why transparency alone does not necessarily repair the injury. Phone access, location sharing, answering questions, and openness can help establish honesty. But those things mostly answer, "Can I verify what you are doing?" They do not automatically answer, "Will you protect me when I am not verifying?"
Before discovery, the betrayed partner may have operated on an assumption of mutual protection. You are one of the people in the world who is on my side. The affair shatters that assumption.
So reconciliation is not just about rebuilding credibility or getting trust back to some percentage. It is about the unfaithful partner demonstrating, consistently and over time, "Your wellbeing matters to me even when protecting it requires sacrifice from me."
That is much closer to what actually has to be rebuilt.