"Or if that is a problem... Simply get married again in Thailand and register it with your Amphur." That does not work. For that both need a certificate clarifying that they are not married!
Usually they do not care bout the real date of the return flight. But the safest and simplest option is a flight where you can change the date. So book it for the
*****
days original return requirement and then change it accordingly to your changed plans.
With Thai Airways that is no problem, no idea about BA, though.