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...
7 months a year in a country for 4 years means that's the main country you live in - i.e. You're not a tourist, you're a resident who travels abroad 5 months of the year. That's what they're seeing. As others have said, a tourist visa isn't likely to appease them.
Probably not. You're spending a lot more time outside of Thailand than inside, you're not even using up half your 60 day allowance on any of the visits... Your pattern says "tourist" rather than "resident." (Especially if you're going all the way back to the UK in between visits.)