I agree about nicknames. Everything you said could be possibly true. But problem is, these knights are mentioned simply as Mikulaš Šraněk and Mikulaš of Reblice, they don’t have Radzigs title (“of Dvojice”, or “of Dvorec”) even after his death, so how they could have the title in 1403?
And also if one of them would be Radzigs brother, I think it would be somewhere mentioned and also his title should be the same as Radzigs, but they don’t have any, or different title.
So the fact it’s Radzigs old father is more possible for me. Or his brother, who is unknown in history and could die sooner than Radzig.