Firstly, 'common law' spouses are not recognised in Thailand. Apply as far in advance as you deem necessary, that will all depend on the turn-around time at the location you apply for the visa.
I can't send you the graphic for applying in country for a Non-B visa (am on my phone) , but yes it can be done. A lot of employers prefer you to go abroad and get the visa, as it is less paperwork for them.
Guessing you are coming visa exempt? If so, it is your air carrier that may ask to see proof of onward travel within 60 days. It does not have to be a return flight.
Wrong, it is not still a retirement visa, I will stop there because your lack of the correct terminology is painful. People like you think you are giving advice, but all you do is muddy the water.