Did I ruin my cookware?
Now you know what salts and minerals are dissolved in your tap water. Happens to us all.
Fill with water ands heat it up. It should go a long way towards dissolving any encrusted residue. If something is really attached, try white vinegar.
Any discoloration to the metal while disappear over time.
Fill your new pan with warm water and a splash of vinegar. Heat it over medium heat for a a couple of hours (verifying it hasn't boiled off much water or add water to top it off). Dump it out and scrub with a scrubby (plastic sponge or Scotchbrite pad) with some dish soap and it should be as good as before.
The color from heat stress will diminish over time.
The powder is most likely mineral salt, like calcium.
Is it possible to overheat a stainless steel pan and ruin it? Absolutely yes. Most stainless steel cookware are really stainless steel cladded cookware. You can ruin the cladding at high temperature. However, given that you said you still see a few drop of water at the bottom of the pan, then you are extremely unlikely to have overheated it -- at least not to the point of destroying the pan.
You are fine.
To remove the white mineral, you can use distilled white vinegar or you can use Bar Keeper Friend -- just like what Sid Post said.