The existing buttons are made out of a plastic that wears well when rubbing against their contact-points in the case. The plastics are chosen to be compatible, self-lubricating and "not spalling or rubbing eachother to death".
I doubt that these metalized buttons have been tested for their long-term wear characteristics.
I like the color of copper, but managing cupric oxide would be a potential headache, especially if it flakes off into the electronics (which it is likely to do on a moving part, like buttons).
You could possibly mitigate this somewhat by running the buttons through a polishing tumbler with burnishing media, but it would give me too much stress knowing that even this isn't 100% effective.
They don't. Take a look at pennies. The problem is that your fingers aren't only covered in oil. There's also sweat, dirt, dead skin, and bacteria.
The other issue is that even if that was true, you're not touching the entirety of the button. The sides and the underside are just exposed to air, and with enough ambient moisture, expect oxidation to show up eventually.
These aren't full metal buttons. They're a hybrid resin and aluminum or copper mix. So less of an issue if it was a full metal button. You can customize it through polishing it desired, e.g., a brushed look, fully polished, raw (as it comes packaged). And yes, your finger oils will add to the end result as well.