You could leave a few days earlier. Or, if that would mess up your return flight, take your gf to Phu Quoc for 3 days before 7th Feb. She'd probably quite like it.
I had it happen once when I was in BKK recently. The girl behind the counter had a lousy attitude, so I reckon that was the reason. No-one else asked for it when I topped up other times.
If you've returned to the UK (since your first Thailand visit this year), then either (A) that'll be registered via your passport or (B) they won't know whether or not you've returned to the UK. Either way, not a problem for you.
Yes, you should worry. Because, when you return in December, you'll have already been here almost 6 of the last 12 months. They might stamp you through. They might ask questions. They might even refuse you entry.
Depends how long you stayed here, and the gaps between your 3 visits. As a general rule, they're looking for people who live here, but pretend they don't. If you've visited for a couple of weeks at a time, then going back to your home country, you'll be fine.
The visa waiver is for tourists. If you want to live here long term, there are visas for that. In fact, when it comes to offering long-term visas, Thailand is unusually generous. Far more generous than, say, Schengen, UK, USA...
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