Stirlingengineforum.com

Running two engines off a single flame is not the same thing - the flame isn't dependant on the output of either engine.

Unfortunately, every type of engine has an upper-limit to efficiency - this is often described as being a "Theoretical Limit" but that isn't really good grammar, as there is nothing theoretical about this limit, other than that the practical limit will always be a little lower.

I really think you should read in to how energy flows and try to understand the thermodynamics behind the numbers you are crunching.
As a first, you only compare the output power from the Inresol generator with the input power to feed the furnice. You totally forget to find what input power the Inresol generator require. If you read their papers you'll find that beside the electrical output power of 5kw it also produces about 10kw thermal power as waste from the cooling circuit keeping the engine able to cope with the 900degC.
It also says that the engine consumes 17kw of thermal power from the heatsource.
So now you have to feed 17kw of electric power with a 5kw generator. Not that funny anymore.