Usually with a state agency such as the secretary of state, with assumed or trade names sometimes registered at county level.
Usually not. EDGAR mainly covers publicly traded companies and certain other registrants, not most small private businesses.
Not necessarily. Some officers and agents are nominal, so confirm operational roles with other records.
Business names change, but the registry's entity number usually stays the same, making it more reliable for tracing a company.
They can help, alongside property and tax records, but confirm actual control and access with other evidence.