Bike porn

... so about 540N is applied to the lower element of the bottle cage, the important (i.e. weak) part of which is the beam under the bottle consisting of two parallel flat steel plates of approximately 4mm x 1mm section and 40mm long, loaded in the worst possible way. ...

I'm not sure why you're focusing on the bottom of the bottle cage? The beam the cablestop hangs from is connected to the strip that runs down the back of the bottle cage. That strip appears to continue to the top of the cage, so would take the load in tension.

I'd be worried about the dodgy join between the beam and that back strip. If the joint from the beam to the connector is free to pivot, you'd get some movement from the strip bending at the top of the connector allowing the pivot to move down a little.

But i'd be more worried about the beam itself and the risk of it buckling sideways then bending down in the middle, pulling the bottle cage backwards as it goes. The connection to the cablestop doesn't seem particularly careful to avoid the twisting that could precipitate failure.

It is an otherwise lovely bicycle though.