Most buses to Hua Hin leave not from Ekkamai, but from the New Southern Bus Terminal (Sai Tai Mai). However, Sai Tai Mai is in far western Bangkok a solid 10 km from the city center and not reachable by BTS/MRT, so you're likely to run into traffic jams just trying to get there...
I know you said "bus only", but I would still recommend the train. Trains ...
Well, it doesn't answer my question but it's quite close; there are vans going out of Ekammai bus terminal in 05:00,
quite close to 03:00 but not the same effect of course:
A permit allows you to enter Thailand, and is valid until a certain date. If you are staying for longer than 90 days, you must notify the immigration bureau every 90 days. This notification is not an extension of your permit's expiry date.
The difference between overstaying and overdue is that overstaying relates to you and your presence in Thailand, while ...
Per TIMATIC, which is what check-in/gate staff are meant to use:
Passports issued to nationals of Sweden must be valid for
the period of intended stay.
It's a widespread myth (not least in Sweden) that airlines can make up whatever rules they like. They can't, because if denying you boarding despite correct documents, they're obliged to re-book ...
Per TIMATIC, the database used by airlines:
Passports and other documents accepted for entry must be
valid for a minimum of 6 months from the arrival date. [With the following exceptions:]
Nationals of Thailand are allowed to enter with an expired passport.
When nationals of Thailand travel with a Certificate of Identity or a temporary ...
It is very likely that you will need a transit visa. According to https://www.don-mueang-airport.com/terminal-2.php Air Asia flies from Terminal 1 and Thai Lion flies from Terminal 2. I don't think there is an airside connection between the terminals.
What 6+ months validity? There is no such rule. Thailand only requires your passport be valid during your stay, and that is only if you clear immigration. If you remain airside, it doesn't matter at all.
It's impossible to answer this conclusively. Prices depend on specific flights, booking time, whether you need check-in luggage etc. For the best price, you'll need to search every time you fly.
City pairs that are geographically close and have low-cost flights are generally, but not always cheaper. For example, Chiang Rai to Kunming; Hat Yai to Kuala ...
It's unlikely that you'll find better options within reasonable driving distance than DMK in general, to fly to nearby countries. As of January 2020, Thailand has 11 international airports:
2 in Bangkok
2 in the north
5 in the south and
1 in the north east.
The one I've left out serves Pattaya. The only neighboring country with a direct connection to ...