Your choice. The marriage visa is more difficult because the Immigration want you to jump though hoops to prove that you are actually married and living with your wife but requires less in the bank and less proven monthly income. My advice: If you have sufficient funds opt for retirement visa.
If you have a history of being denied entry the a tourist visa from a Thai Embassy is the best option. But have 20,000 Thai baht cash in any currency, an outward travel ticket and details of accommodation booking. These are not always asked for but Immigration can require them.
Need a doctors certificate stating that patient is to sick to walk or travel to immigration. Then you can do it for him. Or look at the possibility of a medical Visa.
I do not think that you can do it via mail. The Stat Dec is a legal document. The certifier is legally declaring that they have seen you sign the paper. I think that it must be done in person. The Australian Embassy does a very good outreach program where they regularly visit Chiang Mai.
. Mate, at the present time, you can go to the Australian Embassy and they will witness a Statutory Declaration showing your income in Australian $ showing the equivalent in Thai Baht at the exchange rate on the day. But be aware that Thai Immigration are putting pressure on the embassies to check that the figures in the Stat Dec are correct. The British Embassy has stated that they are unable to verify the income statements and will cease to certify Stat Decs in December. I understand that Immigration have been requiring US citizens to prove the figures in the Stat Decs certified by the US Embassy and that the US Embassy may also cease to certify them in the near future. I expect other Embassies to follow suit. Australian Embassy? Who knows. In the meantime I will continue to use a Stat Dec from the Australian Embassy and I can prove the income figures because all my (Australian) income is in the form of superannuation.