Here is your break down. As already stated all brokers are covered under SIPC. FDIC would only apply if you have a bank product like a cd and certain money markets.
1) SIPC, covers 500k cash and secruties, max 100k cash.
2) The assets in the account are yours, since your talking about a retirment account they would not be allowed to lend out stock against it. So unless the firm is engaged in outright fraud your assets will just be moved over to another broker capabale of holding the funds. Like FDIC, SIPC only covers the banks/brokers failure. So any investment in that companies stock or funds that lose value ralted to the underlying companies performance are not covered.
Your brokers are required to have on hand and provide brochures/info on SIPC coverage on demand. Next time you talk with them I would request information.. that should at least get you something in the mail.
BGF