Surely anyone couldn't actually challenge any accusation against them by combat, right? Also could people really have a representative fight in trials by combat? What's the point then? To just have two mercenaries fight doesn't make any statement about anybody, does it? In what places and time periods did trial by combat exist? I'm considering writing a short story and I'm wondering how trial by combat would actually work.
Hello there! More can of course be said here, but this and this are both answers of mine which might be useful for you.
As you say you're writing a story, I would also note that while a bit old, "Trial by Combat" by George Neilson is out of copyright and easy enough to find online for a more verbose source.