SPOILERS:
I'm not sure if anybody else has said this, but I feel like Liz is the Sikorsky Archive. Maybe the government or somebody stored the information in Liz (idk: maybe in her DNA or her memory). This would explain why Dom and Reddington (whoever he is) will not tell anybody the truth.
In Katarina Rostova: Conclusion, Dom says that Katarina took the archive. It would be because the archive was in Liz herself, so when she took Masha away, she took the archive. This would also explain why Dom says that both Katarina and Reddington have or had the archive because they had Liz.
I don't really know if N-13 is actually Reddington because Dom was acting delusional, but I think that he is truly protecting Liz. I'm not sure what reason he would need to kill Katarina after she found out, because she would just protect Liz also, so this theory might not pan out.