Why does it say "El Mitzcho"? It should say Al Mitzcho!
Radak: This is like Al Mitzcho. We find "va'Yamlichehu El ha'Gil'ad" (Shmuel 2 2:9), "El he'Harim Lo Achal" (Yechezkel 18:6).
How did the stone enter his forehead? He had a copper helmet. Surely he had a protection on his forehead. His entire body was protected!
Radak #1: Some say that when he said (44) "I will give your flesh for the birds", he looked up, and what was on his forehead slipped back, and then David slung the stone and hit him on the forehead. This is like a Drashah.
Radak #2: We need not ask how it happened. It was from Hashem!
Radak citing Shochar Tov 78: This is one of the places in which Hashem made the soft (stone) overcome the hard, i.e. the iron 1 on his forehead.
Radak #3: Perhaps there was a small exposed area on his forehead, and Hashem caused the stone to land there.
Our verse of the Midrash does not say 'iron.' His helmet was of copper (verse 5). (PF)
Why did he fall forwards? He should have fallen backwards due to impact of the rock!
Rashi (from Shochar Tov 18): This was to save David the toil of going an additional 12 Amos and two Zeresim - twice Galyus' height - in order to cut off his head. 1
Radak: Normally, one who is stricken and cannot stand, he falls on his face. Malbim - had he fallen from the impact of the rock, he would have fallen backwards. Rather, he fell due to weakness.
Radak (from Shochar Tov 18) #1: The mouth that blasphemed should be buried in earth - "Tamnem be'Afar Yachad
Surely David cut his neck. Really, he saved only twice Galyus' height up to his neck! The Midrash mentioned twice what is written about his height (verse 4).
