I can't really believe he'd lie under oath.
I imagine it would be very much non-answers. He'll try to beat around the bush a lot to avoid actually answering the question. I'd imagine that VG lawyers would soon realise this though, and quickly jump to squash it.
I think he'll just do some sort of statement for his lawyers to read out, and not actually show up (will he be allowed to do that?). In his situation, that will probably be for the best (his famous interview alone shows us that) - I think the public will view it as an admission of guilt though, if he doesn't show.
Why was he alloed to get away with so much, for so many years? Shouting at staff, and allowing friends to walk in and out of palaces without signing in? Was it just because he was the Queen's favourite, or were all the family allowed to follow the same low standards, and by-pass security policies? I'd be interested to know if Edward's, and William's and Harry's friends (as W&H grew up) were allowed to do this? Even Charles'?
In which case, a bigger question needs to be asked about the security system the RF has in place. Aren't we always told about how high risk they all are, as a justification for paying for security?