Gary,
Unfortunatley I can't give you too much advice on how to fix EMI problems, because it depends on the properties of the EMI noise and I have no knowledge of where and what type of EMI is your system is suffering from, but usually it is bad grounding, e.g. ground loops in the grounding or just too long and thin ground wires or it can be also that parts in the system just do not have proper EMI filtering, so they emit too much noise.
In the EU there are strict rules for EMI control which manufacturers have to comply if they want to sell electric products, especially power electronics, like motor amplifiers/drives etc.,
but for example in China there are no such rules at all and funnily manufacturers just print a CE on their products as if it was just a nice looking emblem or something.
So, it is possible that your drives or VFD emitting too much EMI which causes the EMI problems.
It is not a surprise that old VFDs had bulky EMI input filters built in while todays' chinese ones often has nothing on the phase inputs, I mean absolutely no filter, I saw many of them...
We have no plans to make a demo mode which runs g-code, because it would open up more gates to code crackers, it would make their job easier, so we will not do that.
And you are right, that when the UC100 looses connection then it looses the position, there is no way to then properly reconstruct the coordinates at this point on the computer side,
because the computer cannot get any more data from the UC100 as the connection is cut and there were lost or broken packets, so it is unknown where the UC100 exactly stopped the motion.
And if the connection is broken from EMI then usually the USB chipset of the computer gives it up, because it suffers from EMI and then to establish the connection you have to unplug and replug the UC100 so the USB connection can be re-established and if you unplug the UC100 it will loose all data it had in it's RAM, so it will no more know where it stopped the motion last time.