I have commented on this facility on several occasions. Tempted to give up but feel it would save a lot of designers a lot of time if updated, which is what Fusion 360 should be about.
a) Offset knows which way you are moving the line, so why is a minus sign required to ensure the offset line is placed correctly. The adjacent dialog box readout is not large enough to show the minus sign so each time you have to lift your head to the main dialog box to confirm whether the direction is plus or minus.
b) Why can't you offset an offset line. Why do you have to go back to the original line each time, this can require a calculation (be it minor) where mistakes can be made.