Why does it say "va'Tashav Na'ami"?
Malbim: When she left Beis Lechem, she abandoned her fields. Perhaps meanwhile, others took the field, like we find regarding the Shunamis who went to Eretz Pelishtim. She needed to scream to the king about her house and field. She strove until she returned to her inheritance.
What is the significance of "v'Rus ha'Mo'aviyah Chalaseha Imah"?
Malbim: Also she had rights in the field, for her husband inherited his father.
It already said "va'Tashav". Why does it say "ha'Shavah mi'Sedei Mo'av"?
Malbim: Had she stayed in Sedei Mo'av, she would have pardoned her portion, especially according to Rus Rabah 2:14 that she pardoned her Kesuvah 1 . Now that she returned, she had rights in it, like we will explain (refer to 4:3:1:2).
Malbim citing a Midrash: She was the first woman who returned from Sedei Mo'av to convert. Until now, they did not expound "Amoni", and not an Amonis (only male converts from Amon and Mo'av may not marry Yisraelim. People thought that also females are forbidden, so none converted.)
This implies that "Shavah" applies to Rus. She did not return to Eretz Yehudah! (PF)
Why does it mention the beginning of the barley harvest?
Rashi: This was reaping the Omer 1 .
Malbim: It explains why Rus went to collect Matanos Aniyim, even though she had a field. She could not seed it; she was poor at the time.
On the second day of Pesach. The verses imply that the next day, Bo'az saw Rus collecting. One may not reap on Chol ha'Mo'ed! One may reap if he lacks food to eat, or for needs of the Rabim (OC 537:15); perhaps all old grain was finished (there was Ra'av, and only now, after the Omer, they could harvest the new grain). Also refer to 2:17:3:2*. How was the entire city there on Nisan 16? After offering Korban Pesach in Shilo (in Yosef's portion), one may not leave until morning of the 16th. Beis Lechem is near Chevron (Ramban Bereishis 48:6), in the south (Makos 9b)! Perhaps they rushed back on fast camels or horses amidst dire need to harvest, or amidst concern for Bo'az' wife, who was close to death. Or, 'the entire city' is all who were there. Rashi's Perush resolves a difficulty; refer to 1:19:2:2*. (PF)