In incompressible flow the Navier Stokes are divided by the density so you end up with kinematic units (kinematic pressure, kinematic viscosity, etc.). If I’m not wrong, even though you can define a density, OpenFOAM will default its value to 1 when in incompressible. I usually stick to coefficients and then calculate the forces using the well known equations.
Regarding your second question, there’s no direct way to do that in OpenFOAM that I’m aware of. You can use bins to get the distribution of forces and then, with a static analysis, calculate the forces exerted on the front and rear wheels as in this project