Is it against the law to have an unassisted home birth if you are high risk?

No, just unwise. It may not be illegal, but I am at a loss to understand why you would take this kind of risk for yourself or your baby. It would have to be unassisted because high risk automatically excludes you from a home delivery by anyone reputable.
Not really. It is not illegal to jump off a cliff either, but the results can be similar.