An extension based on retirement using the embassy income declaration is the easiest extension there is. Is your embassy still issuing the income declarations?
Please stop using the word "notary" for the embassy income declaration letter. The letter does not need to be notarized, nor has the consul notary powers. Consulates check ones income and issue a letter confirming it.
haven't heard; read or experienced that. There is no rule stating that the money actually needs to be transferred to Thailand. But then TIT, some IO might think differently.