1) In my opinion, all of the sunshine markets are similar, if not the same. I usually go to the Fri Lihue one, but also sometimes to the Monday Koloa or Tues. I usually see all of the same people at them. Here is a schedule:
Kauai Hawaii Sunshine Markets : fresh island produce. Usually there is a Monday Lihue one, too.
I have never haggled, nor have I ever been with/seen anyone haggle (I am a little oblivious tho...). The prices are VERY cheap and I do not find a need to try to get a lower price, plus I enjoy supporting these extremely nice locals.
2) Here is a list of the recycling centers on island:
Kauai County Recycling - Hawaii Bottle Bill - HI 5 Redemption Centers Map
It looks like the closest one would be in Kapahi or Lihue. My recycling center in Lawai is always open per se, but only pays out the Hi5 on certain days and times.
3) I buy comp stuff at Walmart or on a trip to Oahu. I bought my comp online at circuit city, however. It matters what you are looking for...
4) I am lazy and go to Big Save in Koloa or Ele'ele, b/c they are closest to me..but Star Market has a better selection and slightly better prices. I never go to Foodland. Safeway can be cheap, but depends on the food items- and you need a safeway card!
Hope that helps!