Sounds very strange to me, while i never faced such problems.
I have found some solutions and i would like to share.
You can create 2 macros. One for starting the plasma arc and one for switching it off.
You put them in your code for starting and stoping the plasma arc.
It's not necessary to place them manualy inside the code. (You can touch the post processor of your nesting software).
The first macro also reads the "piercing time" from a DRO and waits until this time ends. Then starts the motion.
Also you create a button which executes this macro.
The program starts normally and can switch ON and OFF the plasma arc at the start and the end of a shape.
Now you have come to a point where you have lost the cutting. Press pause. Hit "run reverse". Hit pause again to stop reverse.
Press the button which executes the first macro. The plasma arc starts at this point exactly where you have stoped.
You don't have to go the start of the line, or start the plasma arc while moving.
So, finaly, yes, Mach3 can "backup on path".
The only "problem" i can see is that the pause button is not immediately executed, while Mach buffers the motion commands.
Hope that helps...
John