Please post your Qs to the mailing list, not to me personally, so
others can comment.
I can't help you with comparing LAMMPS to Mat Studio. I suggest
you turn on/off one term at a time for a simple geometry and see
if energy and forces (pressure) agree. If you study the input coeffs
for the various potentials you should be able to figure out any differences.
Re: forces on fixes atoms, commands like this
fix 1 wall setforce 0.0 0.0 0.0
thermo_style custom step temp f_1 f_1 f_1
will print out the total force on the frozen wall atoms (before
LAMMPS zeroed the force).