Your argument seems to be contradictory, puzzled, that he can be sued and prosecuted and the RF can't stop it. But he won't ever see the inside of a courtroom in the US because the RF will/can stop it.
You accept that PA can be sued. Indeed he has been sued. The outcome of VGs case against him was in her hands and not his. She decided to settle. She could have forced him to trial if she had wanted to and the court Judge, Lewis Kaplan, had ruled in her favour throughout PA's objections so there's no reason to think she was being prevented from doing so.
I believe she can't sue in the UK because she's out of time. That would apply to everyone not just the RF.
The UK police are not pursuing criminal charges which I'm sure people would say is a cover up. Except the FBI have not charged him either. Indeed they've not charged anyone. And as you've said the RF don't have sufficient influence there to sway those decisions,so maybe it's because they don't have enough evidence. Just a thought.
There is absolutely nothing, other than your own personal belief, maybe tinged by your disagreement with the institution of monarchy(?), to suggest that he is not in court because he's a member of the RF. Nor, if he was charged, that the RF could prevent a court case from proceeding.