see my comment on RED above. word of advise please book your accommodation fast. somemore you plan to stay in Myeongdong area cause room there usually very fast fully book.

How long do you need to hike mount Seoraksan?

if you are first timer, stay at Myeongdong area.. Hongdae if far from attractions and yes it's youthful place as many universities is located nearby. both area is very crowded but i can tell you Hongdae subway the most crowded and packed 

you can stay in guesthouse. for airbnb you need to choose very carefully the host. the link you listed, 1st and 2nd not really a good choice based on less review and also not super host. 3rd one is ok but it's not in Myeongdong area. It's in Namdaemun. walking distance to Myeongdong depends on which location your house is 

of cause stay near to subway is better. save your time and less walking. imagine after a day out, when you wanna back to your guesthouse and have to walk those hilly road with all your stuffs. very tired you know..

S.K alot of places is hilly and when Korean tell you it's by walking distance it's actually quite near for them (they used to walk) but not for Msian standard hahaha

www.moneymaster.com.my

Money changer rate at Mid Valley

see my comment above. just my suggestion only. btw, gwangjang & dongdaemun is nearby. cheongye stream & gwanghwamun is nearby 

Day 1
Myeongdong
Namsan Park
N Seoul Tower
Gwangjang Market

This one you need to timing correctly. After dinner at Myeongdong then go N Seoul Tower to see night view.
Namsan Park is almost = N Seoul Tower since N Seoul Tower is actually called Namsam Tower sometimes

Day 2
Hongdae + Trick Eye Museum
Sinchon
Ewha
COEX Mall + Aquarium
Gangnam

Skip the Trick Eye Museum, it's a tourist trap.
Can skip COEX Mall unless you wanna do shopping. However, if want shopping, Gangnam and Shinsa-dong 신사 are better places IMO.
It depends on your "style" also. Abercrombie and Fitch has a shop at Shinsa and other foreign brands also at Shinsa area.
Here you also might want to reconsider the timings. Morning at Gangnam area, then Evening/Night at Sinchon, Ewha  then Hongdae.
Day 3
Gyeongbokgung
Gwanghwamun
Cheongwadae
Bukchon Hanok Village
Insadong

This is definitely too packed. Unless you plan 1 hour on each places.

Day 4
Lotte Mart
Namdaemun
Itaewon
Noryangjin Fish Market

Better merge Namdaemun with Myeongdong since both are quite close and go to the Cheonggyecheon stream/river too.
Itaewon better for at night.
Noryangjin better during Winter ( not sure when you are coming though )

Day 5
Namisan Island
Petite France
Dongdaemun
Cheonggyecheon Stream

Dongdaemun, Namdaemun, Cheonggyecheon, Myeongdong are same area.
While namisan, Petite france is outside of Seoul.
Better whole day for Nami Islan and Petite France, then if you still have time at that day, then only go some other places.
Day 6
Back to KL
For shopping,
3 main types of brands:
Not very cheap, sometimes cheap but Korea/Taobao clothes: Dongdaemun / Hongdae
Cheap branded ( Zara, H&M, Who A U, Uniqlo, etc) - Myeongdong, Hongdae
Higher-end brands - Gangnam / Hyundai Department Store / Coex mall

Hongdae is mix of everything. So, I would suggest spending more time at Hongdae.
They have cheap Korean local/taobao clothes, to branded like Bershka, Zara, etc, and also a MCM store in Hongdae
Many restaurants there too.