Interesting project! It was fun to follow the poker build so I'll be watching this one as well.

I think he'd need a $100K whale investor if he wanted to build that one.

0.03" is probably fine. That was the size I ordered when I built Undertale. It's not quite as solid as the 0.048" that I used for VB, but I suppose the easier bending is an advantage.

Just watch for unsupported ends, which will spring back a surprising amount when hit by a ball. The art scratched off this plastic is how far the Hotland passage wall bounced inwards from random ball hits. About 1/4 inch. However, it did still keep its shape; if it wasn't tight enough to damage the plastic, it would have been all right.
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I love your lane switch design, especially considering its similarity to something I thought of but never prototyped. My idea would have been an eddy sensor version but still with the built-in light.

I was briefly browsing the Mouser catalog to see if I could find anything of value, but didn't end up seeing much on that or how to actually design an eddy sensor circuit. It seemed like there are quite a few supporting components needed, which gave me some pause. I do wonder if it would be feasible to build a more focused sensor from scratch (along the same lines as the trough sensor in TZ) with some magnet wire and a 3D printed spool. Maybe throw on an ATTiny and have it watch an analog input.