You need to cover the full time of (planned) stay. So it's not about the visa, it's about your booked flights at the time you apply for the Thailand Pass. If you plan to stay two month then the insurance have to be two month.
That's exactly what they've told us in different offices too... The witnesses have to be Thai (also for Foreigner/Thai marriage), should be acquaintances, if possible. And the rules "should" be the same for every office. But hey... we talk about Thailand... ;-)
Well... right... "supposed to know you" is also wanted, but they not really care about this that much. But for the other thing... the office at Phuket told us to bring citizens, foreigners are not allowed. Same information comes from other offices where we've asked. If you know different it seems to be a "local" requirement. Same as every IO make their own rules...
What I understand... the question was if he (UK national) can be a witness for his stepdaughter (Thai). I think he can't, only citizens can be witness.