Tooth enamel is ~5 on mohs scale. Quartz, the most common kinda rock (afaik), is ~7. You're correct that enamel is harder than steel though, since it's ~4? Disclaimer: all of these numbers are from a quick google search.

Fun fact, this is why we know ancient people used to make bread - the way they separated wheat from the chuff included dropping it on the ground, which led to sand being in the bread and consequently destroying their teeth over a lifetime of eating it.

It's not a "no one asked you", but it is an idiom. As you can probably guess, it means more "don't help people working against your interests".

So, you'd say something like that to a pro-ICE minority person, for example.

Well, rocks in toothpaste would be very bad because they'd be harder than your teeth.

There's fungi that are eating the pacific trash patch, but the issue is that there's not much energy to be gained from plastics we use. It's slow.