Just trying to be helpful, and I'm in no way an expert, so please take my replies with a grain of salt:
1) I should think that if it's set up in the UK, then it's a foreign account. When you opened your account, did the paperwork mention the FCA (UK) or the SEC (US)? Or perhaps more obviously, are you paying for purchases in £'s or $'s via their platform.
2)Sorry, no idea!
3)My understanding is that as long as the funds in the ISA are actually registered with the SEC and have reporting status with the HMRC, then you should be fine. That seems to be the consensus here and on other boards I've visited. Of course, Uncle Sam will still want his CGT on those funds, even if HMRC has said she's not bothered.