You are grossly overthinking this. Various companies have used private aircraft under private rules throughout the years for many uses that would qualify as a private operation in which way more that 6 passengers were on board. The reason the cap is 6 for cost sharing is that those pilots who like to 'undercut' the industry would come up with ingenious ways to build hours for next to nothing flying Chieftains etc on regular routes charging half of what an airline does. Even making a bit of cash with dodgy accounting. Basically what Uber is doing to the taxi/hire car industry. 6 is a nice limit to say, yeah get some friends and hire a plane for a holiday share the cost no problem, good chance they are all aware that its a private flight and know the pilot and that its not some quasi airline operation.
And note well: That flight could still fall within the scope of a ‘cost-sharing’ flight under the current rules, because the definition does not set a maximum POB number. It sets a maximum seat configuration number. Maximum seat configuration, including the pilot, of 6, 6 adults including the pilot and 1 babe in arms by my maths equals 7 POB.
That's one smart infant to have enough cash to contribute to the flight willingly.
But here's a link to another part of law possibly to come in to effect; Why is 6 seats cost share acceptable safety wise, but 4 seats will be say a limit on drivers licence standard medicals. But I can drive a 12 seat road vehicle on a standard drivers licence and medical... That's where the arbitrary nature of these rules start to stand out. I'm allowed to jeopardize 12 people on the road, 4 people in flight for the same medical requirements or 6 people if i want to cost share, 2 people if i want to build my own small plane and so on....