Past plans for the future

We agreed that, when xyz happens, we would do abc

This sentence does not make sense to me. If ‘happens’ is present tense, xyz clearly is still in the future (or is a general statement, ‘when’ meaning in effect ‘whenever’). Therefore, ‘do’ should also be in the future (or present) tense, rather than in the conditional mood:

We agreed that, when xyz happens, we will do abc
We agreed that, when xyz happens, we do abc

Having ‘happens’ in the present, but ‘do’ in the conditional creates an impossible time line, and the sentence collapses semantically. (Although in colloquial language use, I’m sure you’d be quite likely to hear it used)

However, since you say that xyz was future when the agreement was made, but is now past, this is not the proper option to begin with.

We agreed that, when xyz happened, we would do abc

I’m not sure what contingency it is you feel is not expressed here. If you know for a fact that xyz has in fact happened at the time of reporting, there is no contingency to express: it is an at the time future event that is now marked as having actually happened.

In general, I feel the sentence would flow better if the latter two phrases were reversed:

We agreed that we would do abc when xyz happened

But that is a matter of style alone.

As for your last question, I am not aware of any particular grammatical term for the interaction between such constructions, but I’m sure someone has coined one somewhere.