Why does it say "Ledaber Lo"?
Rashi: It is for him (Adoniyahu).
Malbim: It is for [Shlomo]. She thought that this will be good for Shlomo, so Adoniyahu will love him when Shlomo fulfills his request.
Why does it say "he put a chair for the king's mother"?
Rashi citing Bava Basra 91b: This was for Rus, the matriarch of the kingship. (She was still alive. 1 )
Radak: This is very difficult. (Bo'az fathered Oved from Rus. According to Yalkut Shimoni (Shoftim 68), Bo'az was the Shofet Avtzan; there were over 130 years of reigns of the Shoftim and kings after Bo'az and before Shlomo. Sanhedrin 95a says that Avishai killed Orpah (Rus' sister) on his way to save David from Yishbi b'Nov, towards the end of David's life. - PF)