For your first question… make sure that you have set the same units in Simbrief and in the aircraft menu. I experienced the same before I found that I had lbs set in the PMDG FMC menu (that’s default set by PMDG). So I tanked only about half the amount calculated in kg by Simbrief.
FMC → Menu → PMDG Setup → Aircraft → Displays → Page 9/9 → Metric/Imperial Units
