Thinking about the solutions -- I think a starting point is to assume that at least a few more people might have gotten out, if they were wearing basic seat belts instead of a full tethered harness. The pilot did. Cold shock would still be an issue, but they might have had a better chance. Especially if they had been given a quick "open your seatbelts and brace!" command by the pilot, during the auto into the water.
The harness is only required because they're offering not just a "doors off experience," but also the option to sit in the door frame with legs dangling. I'm not sure why everyone has to wear a harness in the cabin, even those seated in the rear, but maybe it's an insurance requirement if anyone is doing that. Or maybe people are trading off the "door sit" so everyone needs a harness.
Anyway, the immediate solution seems simple to me: Regulate these flights so doors can be off, including the usual requirements for no loose items, but keep the tourists in their seats with basic, quick-release seatbelts. It can still be "exciting" to fly that way. I've done it plenty of times on photo shoots where a harness and getting out on the skid isn't required for a wide angle shot. With today's self-stabilizing consumer cameras, you can get plenty of nice tourist photos without sitting in the open doorway. That's strictly for the "thrill" angle that these flights offer.
I'm not sure how this could be regulated while still allowing professional photo and film crews to operate as usual with harness when required. But it seems at least marginally safer for this kind of tourist flight. For the specific NYC zone, a lower limit on water temps in the East River might also be required.
I think twins vs. singles is a red herring, because this type of flight will still be offered worldwide in singles due to operating costs. The same issues of quick egress apply over land, due to possible fire hazard in a forced landing. Tourists just have no business being in these harnesses, or hanging out in an open doorway for kicks.