You are mistaken if you believe that Russian society isn't already enthusiastic. But this isn't a country with the warrior culture US has, dying for your country isn't cool in Russia, it just means that you were too poor and/or stupid to dodge the draft.
> There's a law for these things. That law allows sanctions but not attacking random civilians.
Which law is that?
> It also states that any possible harm to civilians must be incidental to the method
Many of the sanctions are specifically targeting civilians, civilian businesses.
> and that any harm be proportional to the possibility of achieving a legitimate goal (kicking Russia out).
Oh no, you're deeply mistaken. The sanctions will not go away even if Russia gets kicked out of Ukraine, they're very much intended to be punitive.