1)
Why does it say "va'Tikon Malchuso Me'od"?
1.
Rashi citing Sanhedrin 20b: He was king (even) over higher beings (angels and Ruchos).
2.
Radak: This was after he killed those who might have opposed him (Yo'av and Shim'i); then "his kingship was solidified" (verse 46).
3.
Malbim: He ruled over all of Yisrael for all years of his reign, unlike David, who for seven years was Melech only over Yehudah.