What race should I genocide

How easy any given class is depends on your playing style of course, but most people agree that valkyrie is the easiest class in nethack. They're good for fighting, they start with good equipment (+3 small shield means that once you've picked up some body armor, you'll have a very nice AC for a starting character) and food and they have a decent quest artifact. They also get speed at level 7.

Race doesn't matter as much, but dwarves are a good choice for valkyries, because they have good stats for fighting and most inhabitants of the gnomish mines will be peaceful towards you (so you can go shopping without endangering your life - as much).

Generally speaking, for an Elbereth square to be active (that is, for it to actively ward off monsters), something must occupy that square. In many cases, this will simply be you standing on it, but if you want to have an Elbereth square maintain its potency while you're not standing on it -- say, to protect your stash, for example -- you need to drop some sort of item onto the engraved square to keep its warding properties activated. Dropping a single gold piece is a cheap and convenient way of doing this, as it's a low-value item, you're unlikely to confuse it for something else, and you're generally always carrying some on hand.

I'm not aware of any reason why you'd want to drop a coin onto non-Elbereth engravings, however, as standard engravings don't inherently do anything.