RS-485 to USB converters (cable conversion)
Is it necessary to do the ethernet cable splicing as indicated in the
  above link, or should I simply be able to use the provided USB cable?


If you can find drivers for the R Pi then you can use the converter. I'd start by finding out what chip is in the propriety RS485 to USB, they might have a linux driver. 


  Is it likely that the shipped cable is configured differently or do I
  simply have no way of knowing that (short of dissecting the cable)?


You will need a pinout and find the voltage, if you have an oscilloscope, it should be easy to find the pins that carry the RS485 and if they are using any additional pins for signaling (some RS485 converters also have GPIO's). By the looks of the webpage, they are only using the two differential pins and you could use any RS485 converter that is compatible with the voltage (which is probably 5V or 3.3V). Make sure the RS485 converter is compatable and find the two data pins and you should be able to roll your own.