Lincoln didn't seek a legal opinion when 5 or 6 states seceded because the Supreme Court doesn't issue advisory opinions. The status quo was that Fort Sumter was a United States military installation. South Carolina was asserting that its status changed because of secession. It was they, in asserting the changed status, that should have brought an action in court to evict the Federal authorities - the Federal authorities did not have to bring suit to assert their right to occupy property already owned and held by them. As it turns out, they decided to assert their right by force rather than judicially.