After some time, Intel-HEX download is now working. HEX file can be downloaded to RAM, verified and executed. This is good start to begin flash programming.

There is some problem in CDC driver on Windows, some data get corrupted, but on linux it works fine, so I will continue my development on linux; and I will return back to Windows later. Probably, some short delay should be added or shorter packets should be sent but it will come later.