1)
How did Avishai help David?
1.
Rashi, citing Sanhedrin 95a: He helped him in prayer. 1 Radak - this is far [from the simple meaning of the verses].
2.
Radak citing Tanchuma Vayetzei 8: At the time, Avishai and Yo'av were fighting Amon. Hashem sent a dove, and Avishai understood that David is in danger. He had Kefitzas ha'Derech (traveled miraculously quickly) and helped David to kill Yishbi.
1
Sanhedrin 95a: Hashem told David to choose a punishment. He opted to be taken by the enemy. Avishai persuaded him to retract, that he will be safe, and rather, his children will suffer.
