Buses to Songkhla pass by the clock tower in Hatyai.

Songkhla is much smaller than Hatyai, can cover it on foot from the hilltop (where a temple is) to Samila beach, where the little mermaid is.

Better to change in Malaysia. Changloon is the usual place, buses to Hatyai stopped here before the border immigration checkpoint at Bukit Kayu Hitam.

If going by train, there used to be money changers at Padang Besar, not sure now... as used to go there before the whole place including the train station was revamped/relocated. The border crossing by foot seemed to be much nearer then, not like 1 hour walk as reported.

Don't have to walk to the town on the other side, as local (Thai) buses from Padang pass by outside the Thai immigration checkpoint on the way to Hatyai.

Not a problem

There would be people waiting there, and also street vendors. Ask them.

If the buses still having bus assistant/conductor selling tickets on board, he will yell out the bus destination at the bus stop.

Counter terrorism measurements?

Last time I was there, no bikes and vehicles parking along the streets.

Hatyai is not that attractive to spend more than a night.

To me it's just a stopover (in my backpacking days) before continuing further to Krabi, Koh Lanta, Phuket or Samui.

Usually it's on the way back home, arriving early in the afternoon, find hotel, then buy next day bus ticket home, and taking the local bus to Songkhla in the late afternoon.

If just for a taste of what is like over the border, a good option would be Betong. Nearer to KL, drive using PLUS to Kuala Kangsar, then the good state road to Pengkalan Hulu, park car at a paid car park besides the border post, take bike taxi to Betong.

There are direct buses to KL, Klang, and Ipoh. These are malaysian 'tour' buses with malaysian plates, and they don't have official bus terminal but park along the streets in central downtown Hatyai to pick up passengers. Buy the tickets at shops with travel agency signs. Ask the hotel receptionist.

(You would also know where they stop on which street if you travel there by bus. )

To/from Butterworth, it's mostly vans. If not mistaken, the van terminal is in front of the train station. Its 2-3 streets away from central downtown, where the clock tower is.

Hatyai's main bus terminal (buses to Bangkok and elsewhere) is quite a distance from the city center.

Local buses to Songkhla passes by the clock tower. Also buses to Padang. (That's the siam town across from Padang Besar).

(I once took a local bus to Trang, before transferring to a van getting to Koh Lanta. It's terminal is a corner shop lot somewhere in the city center, near the wet market.)