DB - a double angle approach is also known as a steep approach i.e twice the angle as the normal site picture approach technique (taught to all pilots). To stay out of Vortex Ring territory you generally fly the approach a lot slower in the later stages to reduce the rate of decent.
With regards to the helipads and oil rigs - what height are the decks at? Are you CAT A for the departure/arrival at the rigs? I understand that the CAA issued some guidance a while back stating the Helideck height should not exceed 60m AMSL? So potentially there's a lot of operation in the avoid curve? Certainly for the older types that you may have flown in 17 years of offshore flying?
Your acceptance of 'Exposure' is potential misguided - just because it hasn't happened it doesn't mean it won't and it doesn't make it any safer. The offshore environment has risks, so does the onshore. If you accept the risk (as stated) then you can't easily say to someone else they can't do a similar thing.
Interestingly if H500 fitted HUMS to his singles (you can now put it on AS350) he would gain up to 12seconds exposure which he could use, perfectly legally, to take of vertically or depart/arrive at a site that doesn't meet the minimum dimensions. He could also fly through the Specified Area.
It would be very interesting if there were statistics on number of onshore movements and H/V accidents/incidents.
Perhaps Helmets should be the next topic (thread creep) - which is something that we have to wear when operating low level, and, potentially in the Avoid Curve.