Typically most shops closed at 9pm.

Yes it’s within walking distance around 5-7 minutes

The answer is not easy, but I will try and explain by an example: If you walk from Indra Regent Hotel, which is in the middle of Pratunam to Phaya Thai station, it will take you 30 minutes. From there it will take you another +-20 minutes by BTS to Mo Chit station (at Chatuchak). At Mo Chit you board the MRT for Phahon Yothin station (the first after Mo Chit) - 5 minutes. Then it is another 5 minutes walk to Union Mall.

All three malls/markets are different in character. Visit all three of them for a good experience. I found staying in Pratunam to be best. Then on Friday morning visit Union Mall, Friday afternoon and evening Chatuchak. Doing this, you get pre-weekend bargains.

I think the styles of cloth between union mall and pratunam/platinum are the same. There are plenty of styles- not sure how you dress. I m 30 as well and I found tons of cloth suit me. If you want to shop with aircon, just skip chatuchak.