I am not a pro, but that sounds like a good idea! I may steal it for some edging here.

That sounds useful! I might avoid rubber because it does degrade slowly in sunlight and some types release some inconvenient chemicals into the soil as they degrade.

Steel or iron wouldn't be an issue. Rust is certainly not a problem, and most good steel won't be an issue either. They make plastic edging, too, but I don't like using plastics where I don't have to. Not to mention that most of them shatter in sunlight after some years.

Irregularly shaped stones usually aren’t a good idea for borders. I prefer bricks or special landscape blocks that fit together more closely than stones can. The bricks or landscape blocks should be sunk into the ground by 1/3 or 1/2 their thickness.

Use steel- the stuff Home Depot sells is fine. Lasts a very long time. I would skip the stones entirely but I like cleaner lines as a rule.