is there a way to open 2 files via command line, where one of them is explicitly specified as read-only and the other one is still editable?
this would be useful for diffs integrated with existing version control systems where left file is in repository and right file is writeable working copy
No, currently there is no such key, although I could add one in future. You can however set RO attribute for one of files before invoking Compare It!, so it will not be editable.