Most big towns have at least one coin dealer. Go there. Don't accept less than "spot price" unless you're in a big hurry. (Find the current spot price for silver online at Kitco.com or Monex.com.)
The "non-silver" items would come out as the "slag" mentioned in the article, and the way to calculate purity is to determine the volume, through displacement, and calculate the density...
When you send out the invite, say "Potluck is optional: NOT required but always welcome." Honestly, it sounds like your friends are not very nice people, though.
I would recommend first making a list of the items you need so you don't get overwhelmed with choices. Then, pick 2-3 websites and compare the quality and prices of...
Sure, but you need to base coat the surface an appropriate color first. Use flat paint so that the leaf adheres well, and make sure that the surface is clean....
In some cases, you are correct. If gold is mixed evenly in the alloy and at a low enough concentration, you can use plain nitric acid to dissolve away impurities....