Not quests, faction based PVP rewards. Once you obtain a given faction with the pvp factions you can buy nicer items from the vendors. The reason for 4 variations is so that people of lower levels can buy the items too.

It looks like the different variations are to allow one to purchase upgrades of the pieces as one increases in level if one completed the quest at a lower level. Not sure how it is actually implimented in game.

I don't know about most of the new items, but for the Zandalar class piece sets it is as follows:

augur = shaman
confessor = priest
demonic = warlock
freethinker = paladin
haruspex = druid
illusionist = mage
madcap = rogue
predator = hunter

As "code literate", it is possible, but not being Alla I don't know how much of a change it would require. If he stores the date items are added or last updated, it'd be a really simple change. Otherwise, it would require him adding a field in the table, updating all items with the correct values, and then writing the display for it. But short answer? Yes its possible, workload unknown.

*EDIT* After checking the items listings, he already has add and update timestamps. It should be as simple as adding a single box to input the "Items newer than this date" text box, and then writing a single query. Would be really useful in limiting the overload, as you suggested.

Edited, Wed Sep 14 12:31:50 2005

In the new patch you can see every item you can get from reputation - even though you've not yet contained the reputation needed, as from boss kills in ZG, these have probably been practicing on test realms, and have managed to go in with good groups able to spend lots of wipes getting to where they got :)