In 8+ years of doing this I've never once used soap on the lags when mounting a TV, but properly centering and levelling before mounting is something a lot of professional installers never get the hang of though, so hats off to that.
Not entirely sure I follow... do you mean you stuffed a box in there to keep the arm in a fixed, extended position? I always just tighten any adjustments before mounting to fix it in position, both to prevent it from moving, and so I can put it at an angle that makes hanging the TV much easier.