Splitting

The smug relief to hear he might be cheating:

I wonder if 'A' or "It is a" might be better than "The " I don't know why but it felt unnatural to me and I had to really think about it.

not me; not us; not yet. We had our doubts
when she dissembled at the Resident's Meeting,
concocting "business trips" for his whereabouts.

That is my favorite part right there. The concocted business trips - in quotes - has a certain level of snark that appeals to me. 

They lose; we win. Is this how you compare?

Who are "They?" I feel like i Missed something there.

Perhaps, if us, you’d merely mouth good riddance,
resigned, relieved; too cold to have a care;
your "C'est la vie" a last indifference.

The line "Your c'est la vie" is really good. 

But if you did ask why, I’d sigh the need

Why not "If you asked why?"

to fabricate a time - a distant day
some random summer hence; when meeting, we’d

I don't understand this line break

be strangers struck beside a river, say,
or under parkland trees; amazed to find
our lost beginning shock us into rewind. 

I like the ending.