Silicone heaters should sustain the stress we are talking about here, but since yours goes directly into the wall plug, I would avoid giving any bright advice about the setup - if something goes wrong, it can cause major damage.

If you don't wish to use the 3 beams method, you could then use 2 beams on the long 470mm side. This way you would have to worry about the bending on the small 300mm side only. With no central beam, the silicone heater will not be mechanically stressed. Beams will also replace the springs, which in my opinion are not a good choice for anything larger then 200x200. You could use spring washers or shims under the beams instead, to perfectly level the beam ends (which represent the 4 corners). Common 10x20 or 20x20 Profile 5 extrusions from Misumi/item/Rexroth are perfect candidates for the beams. Or you can just craft your own if you having a good fly cutter or rectifier machine.

A method for switching between setups is kinda hard to suggest since I am not familiar with your machine. You will probably have some bolting spots near the center of your current router table.