“should be easy” will go down in history. Absolutely nothing wrong in trying. How far that will go will be subjective. So far this month 5 people have managed to do so in Phuket. I guess way more have just bounced out and got another 180 days.
The first report after entering Thailand needs to be done in person. Subsequent reports can be done online. But for the DTV that generally means most if not all will need to be done in person as you have to leave by the 180 days (unless trying to get the golden extension ticket )
Surprised that you got to the immigration officer without the TM47 in the first place. Did you speak to one of the volunteers at the information desk when you got your queue number.?
A lot would depend on your passport country. Your name suggests that you are probably from one of the 93 countries than can enter visa exempt and get a stamp for 60 days.
With at least 15 days left on a valid stamp (some offices 21) you can convert to a in country Non O based on marriage but it will depend on your marriage date. Having a girlfriend won’t get you that visa.
A one way ticket will be up to your airlines policy. Some will insist on a ticket out within the number of days your entry scheme allows, regardless that you could get an extension or change to another visa type. Some don’t seem to care.
I believe the “End of restriction” sign was an import from the UK.
When entering a speed restricted zone like a town or city then there was set signs showing the speed limit - I.e. 30 mile per hour. When leaving that zone they used this sign that basically said you could now drive up to the “national speed limit”. The idea behind that was that if the government decided for whatever reason to change the “national speed limit” they wouldn’t have to change millions of signs around the country to the new limit.