I have concerns because the product descriptions and images don't say the amount anywhere or how long it should be expected to last and under what conditions - kinda feels scammy.

For example, is it 1oz or 20oz? Honestly, it looks like a tiny sprayer - maybe 1oz? How far will it go? Will that 1oz. treat 1 sink/toilet or 10? How long will it last?

I did see that the article says "...researchers also predict the coating could last for about 500 flushes". OK it "could" last 500, or it could last 1. The real question is how long "did" it last when you tested it and what should a normal person expect? Does urine affect it's longevity differently from poop? What happens if you use a toilet brush on it, or use various cleaning chemicals - will that shorten it's usable life?

e.g. from https://www.spotlessmaterials.com/store/p10/Sink_%26_Fixture...:

"Sink & Fixture Coating SKU: S008191114 $15.00 Our Sink & Fixture Coatings are popular amongst our early users. These coatings can be applied to any ceramic surface and are specially formulated for high-touch ceramic surfaces like sinks, toilet exteriors, etc. After application, you’ll feel the difference. Our coatings are ultra smooth to the touch. It’s that same smoothness that makes the surfaces easier to clean. With these coatings, cleaning ceramic surfaces required less time, effort, and cleaning chemicals. Do not apply coating to floors -- it is slippery. This coating can be refreshed with our Maintenance Spray."

I also have the same concerns about your product.

Likewise. At massive scale, conserving water by adding more artificial/industrial runoff doesn't seem like a good approach, at all. This version of the world is behind us.

Given that the goal of this product is to reduce the water usage, it would mean that you not only add something to the water, but also reduce the total amount of waste water, thus increasing the concentration.

Can any remains be sorted out at common water treatment facilities?

Generally small particles like micro-plastics are too small to be filtered by treatment facilities. My assumption here would be no. It could be that a chemical/heating process in a treatment facility could accelerate the breakdown. That's just pure speculation though since we don't know enough to judge.