[HeyLaughingBoy]

You're overreacting!

An isolation transformer would be of absolutely no use here and the optoisolator would provide the same functionality that the existing relays on that board do.

I can't see how the PC board is laid out so I can't verify if there is inadequate separation between the control side and the isolated side of the relay (even so, you only need about 5mm separation to be minimally safe at 120V), but barring that, it's fine!

Yes, this particular device is not explicitly designed to deal with all the hazards of switching a high voltage load, but I don't see anything that leads me to believe that it's unsafe for that purpose.