At least at the Norwegian embassy in Bangkok you’ll have to provide your complete tax settlement from last year or annual statement from your pension provider and proving your income to get a affidavit letter. I’ve never heard of any problems regarding this as it’s used by people staying in Thailand for decades, but as mentioned some immigrations might require a legalization for your first affidavit and you also have to show the original document provided to your embassy.
Some immigrations require a legalization from the Ministry of Foreign Affairs for your first affidavit letter. You normally also have to show the original documentation you provided to your embassy.
Yes you can apply for a 90 days Non O visa based on being married to a Thai from your country. You can probably apply for the initial visa by proving income minimum 40K baht a month but you’ll need to prove 400K baht in a Thai bank account two months prior to your application for extension of stay in Thailand.
You can apply from home just with income or funds as Lester explains and also in Thailand if your embassy in Bangkok offers to give you a affidavit letter regarding income.