I dont think that cVnav is necessary. Why do you think that exactly?
Like I said before, as long as the meridians are disregarded (they are now in 1.5) we are fine. Since all calls, rulers, comms, etc. will be based on the grid, and the magnetic will also be based on the grid + - declination.
So the only real problem here is that in this scenario we will never be able to use real charts, but in game it works fine.