Is it easy to flag down a songthaew in Chiang Mai at night?

Is it going to be the same 20 baht fare if i take a songthaew at 9 or 10pm?

Are songthaews and tuk tuk are operating at night as the taxis?

I haven't lived inside the city for decades, but I remember songthaews and tuk-tuks and even saamlors being available along the moat at all hours, with some drivers sleeping in their vehicles. But outside the entertainment districts and the late-night markets it might be hard to find transport in the wee hours. But in the evening, during the hours you mention, transportation is ubiquitous.

A songtheaw at 9-10pm will usually be a negotiated fare.

Seems like right after dark, everything is negotiated. It can be downright ugly if you are on MM, LK, or in front of Spicy. Walking home is usually pleasant, or find a lady with a motorbike.

Most of the drivers on the road after 10pm are still pretty fair. Maybe asking for 80 baht or so. What you want to avoid are the guys with shiny vehicles and alligator smiles who wait outside Hotels and tourist centers like Thapae gate or Chiang Mai Night Bazaar. A Taxi Mafia. I asked one for a 2.5km ride in broad daylight after he had just despatched his cargo of Chinese tourists. He demanded 200 baht. I hissed at him "one person not 10" and rapped my knuckles on the roof of his truck. Lucky for me I was not beaten with a tyre iron to within an inch of my life for the uncharacteristic outburst.

It is unfortunate but (some) Thais who deal with foreigners too much tend to become greedy.

In prior years, it was rare I could find a songteaw after about 9PM but nowadays it seems there are quite a few running around almost to midnight. Maybe I've been lucky but most of the ones I've used between 9-11PM have charged me the regular rate (20 baht) but, then again, almost all the time I'm going to "regular" route locations (Thapae Gate, KSK, Kad Luang, etc.).