1)
What is the meaning of "v'David Higid"?
1.
Rashi: Someone told David.
2.
Radak: The Lamed is omitted, [and also 'ha'Magid']. It is as if it says ul'David. This is like "Hayu Ven Sha'ul" (2:4) and "Kol Eleh Bnei Machir" (Divrei ha'Yamim I, 2:23). Also "va'Yaged l'Yakov" and "va'Yomer l'Yosef" (Bereishis 48:2, 1) are missing 'ha'Magid'.
3.
Malbim: David noticed that Achitofel was not with him. He realized that he is with Avshalom; David told this to his servants.
2)
What is the meaning of "Sakel"?
1.
Rashi: Ruin. This is like "veha'Sakal Yarbeh Devarim" (Koheles 10:14).