... are you saying that my interpretation is incorrect too? I see no reason why it should be.
Parts of it. I have copied your post below,
with my comments in blue.
We have a girl then, and "it" is her father's marriage that happened in the past. She used "would" denoting unreal future.
As I have suggested, she used "would" with reference to the past.
"Her last term", whatever it is, had not yet come, because if it had already passed, she would have said,
It would have to have happened in my last term. [...]
This is a possible utterance, but it has a different meaning from the one I proposed.
The marriage would not happen in her last term, which would come after the time of her utterance, because it had already happened before she said that.
But it did happen in her last term, the term in which is now studying.
For example:
(I will change "in my last term" to "tomorrow", because I don't know what the last term could be.)
"Your father married Joan yesterday. Why weren't you there?"
"It would have to happen tomorow. I was planning to visit Dad, but I had no idea he was going to marry her this year. If he'd told me, I'd be there now."
I think that here the only natural response is: "It would have had to happen tomorrow". I know we normally associate this structure with a past counterfactual situation, but it is possible here. "It would have to happen tomorrow" allows the possibility, however remote, that it can happen tomorrow. We know it can't, because it has already happened.
Your response would be natural here:
"Your father is marrying Joan tomorrow morning. Are you going to be there?"
"It would have to happen tomorrow afternoon. I have a hospital appointment in the morning."
ps. Note to learners reading this - we are discussing fairly uncommon situations here. Please do not get the impression that "would (have)" is normally as complex as this.