the plan looks okay but here are some suggestions.
try to go to the attractions that are nearby on the same day. if not for a first timer, seoul subway can be a little bit intimidating.
you might want to spend 3 days in jeju instead if u r using public transport. most of the stuffs that you can find in jeju can also be found in seoul. visit the lotte mart if you have the time.
to get to nami island, see this site: http://www.namisum.com/CheckENG/CheckENG1.asp think the view is nicer from may onwards. not sure.
is this the first time that you are doing hiking? if it is, after the climb you might be too tired to go to Techno mart.
the entrance fee for korean folk village is 11,000won, the most expensive attraction that i've been to thus far. not a bad place, see all the filming spots in the korean dramas. btw the korean folk village and suwon fortress is not in seoul per se. it's at gyeonggi-do. you can either take a train or bus there. you might want to shift it to an earlier day.
you might also want to keep the shopping to the last few days. I'm assuming that you are staying at the same location and dont have to carry your luggage around that frequently. if you are interested, can always go for DMZ tour too.
Hmm... Jeju is more of a 3 day 2 nites. Planning on hiring taxi to get around the area. That is possible right? I read it somewhere on the tour 2 korea site.
Nami island, I know how to get there
Uh oh.. First time doing hiking... Hmm...
Yup, read abt Suwon fortress on the tour to korea site, Im planning on following the sample itinery, which would means starting the day departing from Seoul @ 9am... Ha...
Hmm, shopping on the last day.. Now that u mention it.. I thk I should.. else, have to carry the stuff around... Will be staying in 2 hotels, one for Seoul, one for Jeju. Nami will be a day trip from Seoul.
Questions abt shopping @ Namdeamun, Myeong-dong, Dongdaemun. Im specifically looking for clothes / boots. What kind of price range am I looking at?

Neither do my travel partner. Haha...