1)
Why does it say u'Mashachti"?
1.
Radak: Sisera's astrologers saw that if he goes to war, he will be defeated. Hashem drew his heart to go.
2.
Radak citing Targum Yonasan: "Ve'Esatev 1 ", like Targum Yonasan of "veha'Giren" (Yirmeyah 18:21).
3.
Malbim: My Hashgachah will draw his entire camp to Nachal Kishon.
1
Lev Yitzchak: The correct text is v'Sabevinun (drawing or pulling), like the Targum of that verse.