Path of Exile Rarity bonuses can actually affect the rarity of the uniques you get

This is not how it works at all. When you chance an item (or an item drops) a roll is made for the Path of Exile rarity tier of the item. those tiers are normal, magic, rare and some unknown number of unique tiers. For the sake of argument lets say there are 3 tiers U1, U2 and U3. suppose wurms molt is U3 and headhunter is U1. if you roll U1 you get one of the uniques on that tier, in this case headhunter. if you roll U2, in this case it defaults to a rare item, as there is no unique on this tier. If you roll U3 you would get the molt. This system is used because it means the chance of getting uniques is independant of the existance of more common uniques, and Path of Exile rarity bonuses can actually affect the Path of Exile rarity of the uniques you get.

You'll have to google. This was explained by Grinding Gear Games when the system changed a long time ago, perhaps beta I forget. You may be able to find a forum post by mark or chris or something. these are linked from the wiki about uniques:

http://www.pathofexile.com/forum/view-thread/485201/filter-account-type/staff
http://www.pathofexile.com/forum/view-thread/488664/filter-account-type/

No what this means is you run a map and pay 4 poe chaos orbs or whatever it is to apply nemesis from zanas map device.