Ok, just for the record: I was able to restore my iPhone, but it was rather painful. Once I have figured how to enter DFU mode, I tried to restore firmware but have had "hang" problems: the update process was freezing at some point. I have noticed that when you repeatedly launch update process it makes more and more progress. So, I was doing "restore-reboot-restore" for several hours until it was able to make it all through. Once I have done my first successful update, the later updates ran is they should without any "hangs".