1)
Why does it say "va'Takam Lelaket"?
1.
Malbim: She did not wait for the workers to bless Birkas ha'Mazon, for we do not join women to a Zimun 1 , due to lewdness. Immediately she rose to gather.
1
OC 199:6: Women do not join to be part of the three needed to make a Zimun, but if men make a Zimun, women answer! (PF)
2)
Why did he need to warn them not to shame her?
1.
Malbim: If she will gather from the [attached] sheaves, do not go to investigate if she stole (took what is not Shichechah), for this will embarrass her, that she is suspected like a thief.