Going in-country on Non-O (marriage) but need total of 120 days.  Wife will not be with me at time of renewal. Can I (1) do turn around at border (Mae Sai), or (2) get a 30 day extension at immigration (still one full day for me in CNX but without all documents as we keep original documents with wife and she will be in the US),  or (3)  should I just take the easy but more expensive way (I am fixed income and not much of that) and fly somewhere else for a few days? My first choice would be to ride up to the Mae Sai region to visit friends and family, but my understanding is that I would only get a 15 day extension and I will need a full extra month extension.

If entering Thailand on a Single-Entry Non-O visa, you could not apply for a 30-day extension as you could with a Tourist Visa.  Without the wife present, you could not apply for the 60-day "visit wife" extension either. 

 

You could leave at the end of the 90-days you receive upon entry, and get a 30 days Visa Exempt entry upon your return.  You can get 2 Visa Exempt entries per calendar-year at land-borders.  The 15-day land-border rule ended when the 2 Visa-Exempt per calendar-year limit was imposed - so you will get 30 days.   Borders to Laos would not be a problem, and some to Myanmar are now OK, as well.  No need to fly - and border-entry at airports is the worst, in any case.

As Jack Thompson said. 

No 30 day extension allowed on non-O, 60 days if you bring your wife. 

9 hours ago, Johpa said:

Can I (1) do turn around at border (Mae Sai),

Yes you can get a 30 day visa exempt entry.

As said there is no extension you can get of your non-o visa entry without your wife being present.