That is a good point. A fixed term CD account can be broken but there is a captial risk if not held until maturity. So it depends on what immigration says in acceptable.
I don't believe you can get a letter from the US Embassy certifying income these days. I have had both Marriage and Retirement Visa extenions. The Marriage Visa took 3 vistits to immigration. The Retirement Visa took one visit. If you don't have the income use an Agent. I highly reccomened Mots.