1)
Did even Binyamin come?
1.
Radak: Yes. They had no more hope for Beis Sha'ul. This was five years after Ish Boshes died. Kingship was Batel for five years. David ruled over Yisrael only after five and a half years, for Ish Boshes was king for only two years.
2)
Why did they tell David "we are your bone and flesh"?
1.
Radak, Malbim citing Mahari: Even though you are from Yehudah, also we are related to you - we are all Bnei Yisrael.
2.
Kol Eliyahu, Divrei Eliyahu (Bereishis 29:14): A baby's bones are due to his father; his flesh is due to his mother. You are our relatives regarding both of these. I.e. also your maternal ancestor (Rus ha'Mo'aviyah) was from Kahal Hashem, unlike Do'eg's claim.
