How do sode attach to domaru or haramaki? I know the haidate just belt around the waist, but I'm lost on how to attach the sode (and the kota, but that's less important because I imagine you can't see it)

With rounded ones like that it doesn't matter as much, but with the more samuraiish-looking hard square ones it's important.

It's only a thought but why not ask Iso or one of the japanese artists? Possibly one of the museums in Kyoto or Tokyo. I've got some samurai books at home and I'll have a look for you when I get home tonight (it's 13.03GMT here) but I don't know if they have the info you want

According to Stone's Glossary, kota are generally attached with three strings. One string connects the pair, the other two tie around the chest. It doesn't specifically say how sode are attached, but the photos of separate sode show two string loops at the top edges. I would hazard to guess that the kota attachement strings are laced through these loops before being tied around the body.