How to remove cantilever brake arm?
After some thought, and experimentation, I will suggest my own answer. 

How to remove

I see two ways:

Without special tools

Use the pipe wrench just enough to wiggle the bushing a few millimeters out. Then use a screwdriver and a hammer to hammer it out.

With a puller

With a rather small "puller". Possibly together with this plate that can close around the boss behind the bushing. The diameter of the rod on the puller might be greater than the inner diameter of the bussing. In that case, I hope it is possible to insert a threaded metalrod into the inner threading of the boss, and then press the puller-rod against the thinner inserted rod. 



The two devices need to be in appropriate sizes, and I was able to find that online in my country. 

How to fix the deformed boss

Maybe I was lucky, but it was no problem at all. I just pinched on the "ends" of the oval, with the same pipe wrench. After my first attempt I was able to slide a new brake arm on, without any resistance. If that had not worked I would have used a sanding tool, maybe a Dremel.