A free turbine can be confusing if you hear it start up. The first and most obvious noise comes from the compressor/gas generator section. But as the Power Turbine comes up to speed it can sound like that "second engine" starting. Listen closely next time someone starts a 206 in your general direction. You'll go, "How many engines are IN that thing??"
As for the fuselage, it would not surprise me at all if Frank installed the RR-300 in a modified R-44 airframe. That's just how things are done. The wider cabin will come later, once the powerplant installation is worked out. The "Goat" in the pictures and videos is likely just a test mule.
Frank will have an interesting time pushing this thing through certification. It'll be easy (uhh, "easiER") if he adds it to the R-44 Type Certificate. But if he goes for a separate "R-66" TC, all bets are off as far as certification dates. Either way, the taller mast and wider cabin will probably require the whole test flight regime to be done and documented again. The FAA is hardly any *less* conservative than they used to be. And, given Robinson's reputation, the FAA will probably be extra-cautious with them.
They've undoubtedly already been flying the mule - either with a C20B or maybe even an old C-18 just to see how it would perform and get some "rough" fuel calcs. Certainly, there have been rumors for over a year that people have been hearing the sound of a turbine coming from the Robinson plant. So they didn't just start on this yesterday. Disguised as a regular old R-44, it easily could have been trucked to a remote location for testing.
Let's give Frank some credit. This isn't going to be some half-arsed, cobbed together abortion. If and when Frank brings it to market, it will give the 206 a serious run for the money - not to mention putting that Enstrom 480 and Schweizer 333 right out of the game for good.