For your first problem:
your fabmaster version must over v8g2,then you can set the corner of four!
for gnd or vcc net,you can't set the fix Nail No to gnd or vcc(#1,for gnd,#2:vcc).then the path is not cross!
For ur second issue:
Maybe your mentor input version is not the lasted version, you can contact Vayo Company for getting your answer!
For 3th issue:
you can send your faz and nail.asc to me,I will help you check it!