I have no idea on what reason the Non Immigrant O visa is issued, but if you have Thai family you may apply for the 60 days Extension of Stay to visit your Thai family otherwise it will be the 1 year Extension of Stay.
No, these double entry Tourist visa are not being sold for many years.
There is now a Multiple Entry Tourist Visa, valid for 6 months and on each entry during these 6 months you get 60 days of stay and each can be extended with another 30 days at local Immigration. With good use this visa can give almost 270 days inside Thailand.
If all document are uploaded correctly it take usually a few days to issue the visa by e-mail. The visa is valid for 3 months and you need to upload flight details.
You can change your flight once the visa is issued but be aware that you must use the visa during the three months of validity.
Sunday is the Immigration Office closed, so no problem to go on monday as first working day after closure. But it could be busy on mondays, best is to go friday and the Extension of Stay is added to your current stay.
Day of arrival is day one. Start counting from there, if you use a visa exempt entry you can apply once for a 30 day Extension of Stay based on tourism.