I use a usb to serial adapter connected to an extended 25pin (going off vague memory here) that goes into the cutter. My adapter came with a virtual serial port driver that has been pretty reliable. That would get you going in regards to your second question. This setup uses a cable that's around 7.5 meters.
Now your issue with the USB could be a number of things. If you've checked the inputs and drivers already, then more than likely, the cable can't handle the data at the length of the cable. I would only recommend high quality USB cables under 2 meters for plotter work. I've not once had issues with that length or less. Any longer and we've seen many odd issues that were only fixed by replacing the cable.
If you're positive it's not the cable then it's obviously an issue with the inputs or the drivers.