I'll focus on this statemenL=:
So seems to me RAW already contains all required HDR information and
additional RAW/JPG photos will not produce better result for HDR (only
worse because of alignment process).
RAW contains either a 12- or 14-bit A/D conversion of the sensor readings. Not quite that simple, but depending on your camera, normal case is 12 bits, better case 14 bits. Think of HDR as a series of n images (where n is however images you want to use to get the detail you need in the highlights and shadows). Ideally, each image's dynamic range overlaps the dynamic range of the next one in the exposure group by a small amount. At this point, I think you'll see that in a 3-exposure range, assuming overlapping by about 1/3 and 12-bit A/D converters, you will still be using some 28 bits of data. Add more exposures and you can expand this up to 32 bits, which is where most HDR technologies stop.
So it is not true that a single RAW file contains all this data. With bracketed exposures, you are capturing extra data for both the low and high end beyond what you could have captured in a single frame. It is, however, true that you have to exercise great care not to move or bump the camera (and hope there are no moving objects in the frame). So there is some image degradation (that is not quantifiable) associated with multiple frame HDR.