Generally, yes, sandboxing would be a defense, because criminal negligence (again, it's state specific, so this is a law-school-level generalization) requires

Generally, yes, sandboxing would be a defense, because criminal negligence (again, it's state specific, so this is a law-school-level generalization) requires "gross deviation from the standard of reasonable care". So a mistake in judging the kind of sandbox or isolation you need would not be criminal negligence unless that mistake fell into the above category. I can't think of a case where it would or has - courts have consistently held mistake of judgement to be below criminal negligence in every case i'm aware of. I'm sure it's happened somewhere though. As i mentioned elsewhere, the standard is basically "total disregard for safety in the face of an obvious and huge risk that resulted in injury or death". I don't think anything we are talking about here comes close to these criteria.