a Steel Box Section Build with SBR20 & Ballscrews Plus a few questions

[EddyCurrent]

Ah OK so if I had a movable spindle on the plate I can just set the coordinates on the work piece and then even thought the spindle has moved Mach3 would still know where to start from, from that same cutting path, I Wouldnt have to reset back to 0,0 and then get a new cut path. to get a continuous cut.

That's right, as long as you tell Mach3 where work 0,0, is you're good to cut.
I've just realised you might be considering moving the spindle in the middle of a job, that's going to give you problems but could be done if planned for in advance, you could split the job up into, 'sub jobs' for example.
I would stick to, one job = one spindle position.