Have you considered leaving Hua Hin the night before your flight and staying near the airport? There is an expensive Novatel right near the airport and lots of cheap, basic but perfectly fine hotels within 10 minutes and many offer free transport to airport. There are no problems, for now, as the airport isn't in Bangkok but about 30 minutes by car outside the city.
If the situation is similar then to what is going on now you'd be better off staying on/near the river if u want to see Bangkok. You can see a lot of the sights by boat and you wouldn't be in the middle of a giant circus if things stay as they are now

Did u mean that u were planning to go to Bangkok at beginning or end of trip? It doesn't make sense to do it in the middle.