Just one point that I would like to clarify. As I understand from your answer that SHIRK is the only thing which Allah will not forgive for anyone, besides that he may forgive any other sin after death. But for a muslim person who is already in the folds of Islam finds out that he had been commting shirk and repents, is there room for taubah(repenteance) of shirk or not for such a person.
The room for repentance is always open for everyone. No one, whether Muslim or non-Muslim, is deprived of this blessing. As for 4: 48 and 4: 116, such verses relate to people who committed shirk and died in the state of committing shirk and never repented or corrected their behavior.
I hope this helps.