I have a deal with my SO, that, if at any time, I feel that either of us is too overcome by emotion to discuss something rationally, we get to say so, the other person gets to say their piece (without lecturing or railing) and then the discussion is tabled.

If either of us feel it is important, then it's up to them to bring it up at a later, calmer time. Otherwise, it wasn't actually that important.

This rule has saved us from all sorts of pointless strife.

In my experience, writing things down brings additional insight, more than an accident.

When I first heard Leslie Lamport say “If you think without writing, you only think you’re thinking.”, I thought it was a bit much, but now I suspect he's right.

"Trust" also happens to be the foundation for a good relationship.

Though that's usually easier said than done. Communicating openly and honestly is something that a lot of couples think they do, but it's hard to tell the difference between a partner that isn't communicative (or was once communicative, but stopped), and a partner that has nothing to communicate. It's trusting communicativeness to self-perpetuate. In an ideal relationship we could have faith in such implicit self-perpetuation, but humans aren't always perfect, and having an explicit and expected opportunity to communicate new expectations (in the OP's case, via a (perhaps deliberately ridiculous) contract negotiation) might be all it takes to keep communication on track. If it keeps them happy, then good for them!