Canceling them does not give you 7 more days. As Robert said, you must apply for, and pay for, a new extension. If it is denied you will get a 7 day stamp.
Medicare is useless outside the USA. You need to check your Medigap policy to see if it is and if it meets the Thai insurance requirements and can provide the form Tod mentioned.
That’s exactly what I did. But you need to get started right away as it may take a while to open a bank account and get money transferred. Chiangmai requires a minimum of 21 days left when you apply for your non O visa.