You are slightly confused.
The single entry you refer to is actually a Non O based on marriage.
That gives you a 90 day stamp.
In last 30 days with minimum 15 days remaining on stamp you can apply for 12 month extension.
Yes you can apply for 60 day extension to visit wife.
You do not need to obtain Non O in Savannahket, you can enter Thailand visa exempt and obtain the Non O at your local immigration office.
Does your country provide "income letter" you could use income method instead of money in bank method.