Here is another optical USB extender. It is more plug and play. One point I noticed is that it is FCC Class A. FCC Class B products would be more appropriate for your application. Class B certified products will emit less and symmetrically, be less susceptible to external RF energy.

I doubt that the IR gadget you found will be useful. IR control is normally one transmitter to many receivers via direct optical. In situations where direct optical is not possible one uses an optical receiver to wire converter and connect the wire to an LED attached to the controlled device. This is a one to one situation, parallel connected LED's are not welcome. Sometimes simple parallel connected IR receivers will work, but one should not assume this. Using the Choice Select device you could have multiple optical receiver devices connected to multiple emitters and receivers and emitters are isolated from each other. From a logical control standpoint all receivers and emitters are connected in parallel. For example; there is no ability to route receiver 'A' to output 'c' only. IR receiver output 'A' is sent to all output ports.

For your application you would need an IR transmitter able to send at least eight unique codes and a decoder device that would key one of eight relays in response to a unique IR code. You could then use a simple "Ir Repeater"/"IR Extender" to send IR commands to a remote (wired) location. You need a decoder box of some sort that can decode IR commands and key the relays. In this scheme there would not be any conducting path between the IR transmitter and the decoder.

I stumbled over this wireless IR Repeater. I have no experience with this device and I doubt that it would work well in your high RF field environment. On the plus side there would not be any wires between locations.

Since you already have a USB scheme that mostly works, all you need is a way fortify it against the high RF field. This may be as simple as enclosing your endpoints in a metal box, bypassing the DC control lines to the relays, and adding a few ferrite beads.