I don't have a good answer to 1, or indeed any answer really. So I'm sort of stuck really. I don't come from a religious background, and nor in the story does Peter. Generally, those I have met who have joined a ministry are people who are later in life whose life experiences have pushed them in that direction, rather than somebody who has suddenly chosen to sign up in their 20s. But I guess there folks who have done this. I'm wondering if to change the story so Peter does not actually joins the church but meets, early in life as an undergraduate, a Canon from the cathedral and they are both main characters in the story, as their friendship develops. Maybe Peter joins the army and still goes to Yugoslavia with NATO or maybe he's there because he becomes a journalist or a charity worker, and Canon X (the new character) still provides access to the Cathedral and participates with Peter in the 'crime' of embezzling funds to rescue Anya (the orphan) from Yugoslavia.
For 2, Peter gets dumped because he is too needy, but also because he is diffident, shy, not exciting enough, not a risk taker (though over time he does change). Catherine is looking for someone that lives more on the 'edge' - she's not planning for 10 or 20 or 30 years, she wants to live in the moment, at least in her early years. She dumps him because she sees more enticing, thrilling opportunities elsewhere even though she sees these as riskier relationships to take on, she wants to leap into the unknown rather than continue with the well known and unsurprising relationship with Peter. She's not a bad person, but she is decisive and forthright, so after the convenience of her relationship with Peter at Uni she dumps him as her life moves on. The pain of getting dumped is what makes the indecisive Peter wake up, react and take a surprising path for him (eg join the Church, or go abroad, or decide to make something of his life in Charity work).