The way GW did it worked great.. but trying to track ratios based on player trades is a bad idea, and completely unnecessary. Currency vendors would naturally balance out to the market price on their own GW-style.

I don't think this is really necessary though. The forum-based trading should make it relatively easy to exchange currency items with other players.

Yes it increases chance of getting multiple drops from same mob.
Quantity that isn't linked to a particular skill (like the gem) will also affect chests.
It does increase the number of drops from unique mobs, but not very much... magics/rares/uniques all have built-in quantity and rarity modifiers, and the quantity/rarity you have stacks additively with it. You only get the full effect on white mobs, which have no built-in modifiers.
Quality only affects the odds of getting magic, rare, and unique items. It has no affect on currency items. It doesn't affect number or magnitude of mods on items.
Having 100% rarity will double the number of magics, rares, and uniques you get from white monsters... and less so with magic/rare/unique monsters due to their built-in bonus that I mentioned earlier. You could theoretically hit a point where you never get white items, but I doubt you can get nearly enough rarity from passives/gear to do that.