1)
Why does it say "va'Tishchati Es Banai"? Verse 20 already said "va'Tizbachim"?
1.
Radak: Because this Avodah [offering children to Molech] is more severe than others, it is repeated. It says Banai, like "Asher Yaladt Li" (20).
2.
Malbim: Now you took sons that truly were mine!
2)
What is the meaning of "va'Titenim b'Ha'avir Osam"?
1.
Malbim: She gave them to the priests. One is liable only if he hands them over to the priests, and takes them from them and he himself passes them [through the fire].