[COLOR=red]Well,if the riot is off then is ok for u to go to BKK by the Suvarnabhumi Apt or arrive at U tapoa.
Day1
11.50 - Arrive in Survanabhumi Airport
         - Taxi to suk11 - check in
14:00 - shopping @ chatuchak
          - Take BTS at nana station to Asok and change to Sukhumvit MRT.. then to Kampheang Phet station -

U don't need to change to MRTA. From Asoke u can the train straight to Mo Chit Station (Last) n when u get down u are at Chatuchak http://www.bts.co.th/en/map.asp.U only have 4 hrs.may be enough for shopping. Bring along a cap. It can be very hot.
18:00 - Taxi back to suk11 (Is it advisable to take taxi at this hour? Will it be jam? And roughly how much is the taxi fare back to suk11 from chatuchak?) 

Yes, is jam. Again take BTS.Cheaper. Forget abt taxi
20:30 - Going to Suan Lum Night market
         - Nana BTS to Asok, then change to Sukhumvit MRT then to Lumphini stadium (Am i correct with this route? Please comment).

Yes, u can or just take a cab .Quite near.Actually u can forget abt Suan lum if u have gone to Chatuchak. Things are more cheaper in Chatuchak and more things. But if u wanna just jalan jalan then fine.
22:30 - Back to suk11. Is it better to use Taxi or take the BTS and MRT? If taxi, how much is the fare?

Take taxi - insist metre.Fare no idea.

Day2
08.30 - Nana station to Saphan Taksin station
         - Exit 2 to head to the chao phraya Express boat

Don't take tourist boat. just take the boat like the locals..may be 20THB only.Cheaper.

09.30 - Take the river taxi and stop at Tha Chang Pier - Walk to Grand Palace

Becareful when u r there, the tuk tuk drivers will walk to you n tell the palace is close for the day due to some important function n wanted to take you for a ride. Just ignore them
11.30 - walk to wat pho , some 1km away from Grand Palace. Then take the ferry from Tha Tien Pier to Wat Arun Pier.

YES

12.30 - Wat Arun
1.30 - I want to go to the chinatown. And it said that i need to stop at the Ratchawongse Pier. So, from Wat Arun, where do i take the river taxi again? Do i need to walk to Tha Chang Pier again? YES

5.30 - back to suk11. Will need to use taxi. Anyone know how much? Will it be jam during that hour?
YES JAM.Take MRTA to Sukhumvit. They are just across the Railway station - Hua Lamphong
7.00 - dinner and shopping at sukhumvit area

Day3
10.00 - I wanna go to the platinum Mall and MBK. Anyone has any idea getting there by BTS? I get different info from the wikipedia and the BTS website. I just need confirmation. I dont want my mom to walk far.

Platinum Mall
- Nana station - Rajchathewi station , is it walking distance? Far or not?
Is abt 1km but no more than 2 km from both station. U decide. But for my mom she is a morning jogger.So no problem for her

I think u need to cover only platinium and pratunam market and is good enough for u to cover that 4 hours

Platinum Mall and MBK is one end to the other and of almost the same road. MBK is near to Ratchdewi station. Once u hv finish Platinum Mall, take taxi as u may lot's of stuff to carry.MBK
- Rajchathewi station to National Stadium (but need to interchange at siam station for silom line?)

Need to be back to suk11. Is it advisable to take taxi or BTS?

2.00 - Back to suk11 to claim our luggage and take taxi to airport and fly back home.
Please help me!!
Give yrself 1 hr to reach airport.
WHILE U AT BKK APT OR HOTEL , GRAB A MAP TO SEE RECONFIRM YR ITINERARY. M RESPONDING THRU WHAT I REMEMBERED.

---

Added on December 2, 2008, 9:10 am
sUK11 BY TAXI COST MORE THAN 250THB BUT LESS THAN 400 THB

yeah. dont take the tuk tuk unless you know the distance and the price roughly. try not to be so touristy about it either. or nego a price before getting on.