These don't get fire blight. But they do get bacterial blight and verticillium wilt. Try looking those up.

Cut off the dead growth and water well.

Bacterial blight does not present like that.....I'd suspect verticillium. You do want to prune off the wilted, dead portion but sterilize the pruners thoroughly after making the cut.

And keep the soil moist but not wet. JM's are not considered to be very drought tolerant and they dislike dry conditions. Exposure to stressors like drought are one of the ways VW makes inroads.

Sorry but that looks nothing like cold or frost damage on a JM!! It is rarely ever limited to just a single branch or two but is more of an overall effect. And dead leaves - these are well beyond "damaged" - do nothing for a tree so there is no point in leaving them or the branch they are attached to present.

I cut the infected branches off right away and so far none others are effected. It may not look as nice as before infections, but still quite reasonable. Anything I can do at the moment or some other time to stop it coming back or just pray? I am not talking about watering it more often or make the leaves wet, etc. Other leaves are looking very healthy at the moment, like nothing ever happened to the tree.

My mistake - it is still developing... not sure if it can survive. I wonder if it is caused by black walnut which is around 35 ft away?

Japanese maples are juglone tolerant so the walnut is not the culprit :-)

If the dieback continues, then I would consider something more sinister, like the verticillum. But it is also possible to get some random dieback during the growing season. Now and again I do see some on my JM's but rarely is it a entire branch......typically just a small offshoot from a larger branch.

Time will tell. If it is Verticillium wilt, there is nothing you can do for it except give in to the inevitable. Sorry.

At the moment more than a half of foliage is gone and 1/2 of the bark turned some kind of black/purple color. I doubt it can survive and the shape after pruning is simply ugly. Could I plant carpinus betulus frans fontaine or amelanchier canadensis in the same hole right away or I should not hurry and let it sit empty for a while or change soil? How long is safe?

If it was VW, both the hornbeam and the amelanchier are resistant, so either should be good go.

What else it could be with a weird color like that?