Let me summarised again for 3 pax..

Air Ticket: - RM1100
Hotel - RM580
Ground arrangement - RM1500

So total = RM3180 or RM1060/pax.. all in.. nothing confusing leh.. dunno how you calculate haha

My personal experience, i don't really like to take tour. Wasting time, the food they provide sucks, stops at stores/tourist shops and no freedom in terms of timing plus only limited of time you can spend in a place. Also, they will bring you to unnecessary place to visit. Up to you though coz you don't need to worry on the hotel and transport since they can provide for u If you and your gf don't mind walking and taking trains a alot, i suggest you go free and easy in Seoul. You can go Jeju Island if you want to sight seeing. Maximum 2 days there bored liao. I've spent 7 nights in Seoul still now enough for me
The train, cab and bus are very convenient. I took the train a lot. You have plenty of time to do some research on where to stay (book a guest house for cheaper stay, clean and nice as well), where to go (everywhere are easily to go with their subways) and what to eat! (which the tours do not bring you to).
On 3G you can roam only. Off your data and just ON you mobile network with 3G service. Then you're able to call or SMS. Most phones when they Off the data they will automatically go into 2G network instead of 3G. Surf net you can use Wifi

I don't think it works with normal phone without 3G, yours should be 2G phone. I also don't know why  But the guesthouse do provide free internet (2 pcs in the kitchen), free skype and wifi. I suggest you bring one that supports 3G network.
Whole trip, including accommodation + flight (RM680.00 AirAsia, i bought last year coz got promo) + expenses, around RM 2,400.00 total for 8 days and 7 nights! haha...and i did not shop for anything, expensive leh, for guys nothing much to buy lar...for girls okay la...expensive but worth it according to my gf because the quality is really good. Cheap or not i leave it to u guys to decide
As mentioned above, around RM 2,400.00
let me continue for ya...made Malaysia's look like shiat

Even you use chinese, no matter mandarin, hakka or even Hokkien, there will be lotsa similar vocab /shared vocab wan and thus, they can understand haha.. just add abit of heyo, hamida at the back can liao haha..

Don't worry, most of the restaurants they can speak mandarin. Some even basic English. Like what Stimix said, their subways and directions/maps are written in english and chinese also. Do remember to take your hotel card just in case you need to travel back to the hotel by cab. It'll be easier. And if happens that you've met non chinese/english speakers, just learn some basic korean words, hi, bye, pork, chicken, thanks, beef, fish, vege
As for the package, i think its a bit tiring considering you have to change hotel and travel quite far. Mt. Sorak is quite far from the city. Everland you'd probably spend at least half a day there already. Day 4 a lot of shopping at their "compulsory" outlets which is wasting time. 1 hour in Myeongdong is definitely not enough! I personally don't like to walk around Dongdaemun, so to my friends as well. very crowded and not nice to shop. Just like Sg. Wang. I still think you should go free and easy to enjoy the flexibility and adventure around the city by your own. That way you have more freedom and also maximize your holiday value and experience (see more things, eat their local food which tour does not provide, travel around their suburb areas and meet more people).
RM 2,400.00 for me. My gf spent more than that because she shops. Only is Seoul city itself I stayed in Namsan Guesthouse 3, recommended  Book in advance so you can get a cheaper rate. Mine was around RM 155.00 per night for two person. Average about RM 20-40 per meal per person, depending what you eat though. A normal sandwich or toast sandwich for our breakfast already cost us RM16-18 per person . Go there don't save on food ler, eat only. Try whatever that they can offer. Money can find later I spent all on their food, especially streets food, barbeque and etc. Shopping, mehh...expensive and not necessary (if you're a girl different lah, they will spend on cosmetics). Enjoy food and sight seeing more value 
Yeah, of all countries i've been to also no prob but Korea seems to be a bit different i don't know why also. I could be wrong but as what we experience is that we need 3G network then it'll be fine.

Sorry, I have only been to Seoul. No idea on Jeju.. I just saw Jeju from the sky nia  Anyway, really depening on your preferences.
Winter if you wan to experience cold freezing temperature and testing the capability of yoir winter clothing. Snowing & white background esp inland Korea.

Spring - For flower lovers haha and mild cool temperature..(Can be cold sometime)...Not sure whether Seoul got Sakura flower or not.. 1st time seeing Sakura flower last mth in Tokyo..really nice feeling for sure  .. You still can see snow sometime during this season

Autumn..Leaves falling, everything turned yellow & red.. given another diff perpective.. Also cool temperarure and late night & Morning can be cold too.

Just choose diff timing going there lor.. haha.. see the diff.. Well I choose Seoul for winter, Beijing for Autumn & Tokyo for spring.. haha.. Enough of the 4 season experience... No way I gonna go North during Summer as I having bad Summer experience in Hanoi b4- 40C!!

Should not only concentrate on Autumn & Spring.. SHould also experience winter too.. My future - Wanna try winter below minus 20C!! haha

BTW, again, wth proper winter clothing, whatever temperature also can wan inc minus 30C but those minus 20C probably must be better than Down jacket liao haha

I bring 1 jacket from here and will buy one in Korea if needed. a trenchcoat perhaps.

I am leaving tonite and arrive Seoul tmrw morning. Early Sunday morning going to Jeju for 02 nites. Return to Seoul again on 24/4. Where are you staying in Seoul? I'll stay in Namsan Guesthouse 

Really?
cause i check just now,i see nothing.
free seat only for kl to Indonesia, Bali, Malaysia Peninsular and Sabah, Sarawak......

But see some flight only RM2... Wow...

KL to Bali? I only see domestic flights

No, can fit in 4 people only.

You have to take 2 taxi for 6 pax. I don't see any van taxi. If you think is too expensive, why don't u guys just take the shutte bus?? KRW5000 for the whole journey. Nami Island/Petite France/Garden of The Morning Calm and return to subway station Gaypyeong and Cheonpyeong. But u must follow the bus schedule. If u miss then susah lor!! Escpecially in Petite France.

Dunno about that. But the shuttle bus we took from Nami Island & Petite France charged KRW5000. We even took the shuttle bus brochure from the Tourist Information Center.

I didn't know got shuttle bus until we arrived Gapyeong station 

If you want to squeze 5 people in 1 taxi as per Mkaz suggestion, go ahead. But the taxi I took, nicely fit the 4 of us. I dunno where the 5th people can fit in.

Very hard to say also. Weather can ber very unpredictable. I saw many cherry blossom when I was there (22/4-28/4). So beautiful. 

i would say table of 4 perhaps..food wise just order as much as u wan lor.
wah cheaper than mine last time..i should have book kumho instead..haha

weird question u have..of coz have lar.. eating in korea is similiar to eating korean food in malaysia wat..