Speed can be changed through the "magic" of gearing (or a belt, if preferred). A little planetary gearset should be sufficient to mate this to a traditional synchronous generator. An inverter generator's alternator could be designed to run at most any convenient speed, of course.
The Mazda engines were not horribly unreliable, at least in the later iterations, if given proper care and feeding. They were not as low-maintenance as a traditional reciprocating engine, that's true. Overhauls were needed relatively frequently (to replace seals, if nothing else). There was a lot of engineering work needed to get seals that worked for a reasonable lifetime, and to cope with uneven heating and hence thermal expansion of the combustion chamber. For some military applications, engine life is not a big concern, particularly when compared with small size or light weight. If it's going into a cruise missile, for instance, the required engine life is measured in hours.
The Mazda engines, and indeed Wankels in general, are not particularly quiet. It may be that this new design is better in that regard if their claims about exhaust gas pressures are somewhat close to correct. It may also not be a strong point, particularly since they aren't touting it as a feature.
It's an interesting idea, and I certainly don't wish them ill in their development and engineering, but I won't be holding my breath for this revolutionary new engine to come and take over the world. There are many relatively easier things that could be done to quiet a typical RV generator with existing technology. In a lot of cases, simply improving the installation and mounting of the generator would make a noticeable difference. Improved exhaust systems would also help a good bit. Water cooling of air cooled generator engines would help some. These all cost money, but much less money than developing a new basic design of engine to maturity.
