I'm not quite sure what the nail does? The Kinematic works like any other kinematic coupler. They don't swing with the bogie but they move to the side under load like any other. Personally I am not a big fan of them but when bogies sit back a bit like class 56 ones do then bogie mounted couplings can get restricted by the sand boxes etc. My ethos moving forward with design for Cavalex is to use bogie mounted couplers wherever possible.
As for the flywheels, they are a standard size used in US locos. There is enough weight in them for what they are meant to do, that being to smooth out any uneven movement in the drivetrain, the power bank on DCC also does this even better, and the interia is built into the decoder settings. I am not sure what inertial run on in DC really proves in this instance, driving on DC should be full controller control anyway, no one is chopping to zero and hoping the train stops in right place on interia only.
I will be the first to admit that our loco is very much designed for DCC with DC being accommodated as best we can with the complexity that multifunction DCC provides.
DCC (and very much DCC sound) is where the hobby is going and DC is kind of like still trying to make modern software to also be compatible with Windows XP. 🤷