Answer:
1. The Framers, who were later known as the Nationalists or the Federalists, were keen in including both state power and federal power. However, their greatest adversary, the Anti-Federalists, were keen to keeping the majority of the powers to the states for fear of tyranny and corruption. One of the main concerns for the Anti-Federalists, stated in Brutus I of the Anti-Federalist Papers, stated that the States will simply be a cosmetic and that their autonomy would be destroyed by the federal government. Naming the states in the clause was a way to reassure the Anti-Federalists that the states are recognized as independent bodies. However, in being grouped together as the 'United States,' it is established that above their state identities, the former thirteen colonies will work together and respond to a federal government, which forms the United States.
2. This was to make sure that no one, in its present and in its future, would ever view the Constitution as an excuse to promote tyranny and authoritarianism. The Constitution was to do a very specific thing: to protect the people. The clauses and the amendments that follow, therefore, are established to be for the common interest and general welfare of the people.
3. The United States Supreme Court is the branch of the American Federal Government that is tasked with unbiasedly interpreting the Constitution, which includes the document's Preamble. Subsequent Court decisions, such as Brown v. The Board of Education (1954) were carried out for the general welfare and the blessings of liberties for the American people.
I hope this helps!