Why did they compare Rus to Rachel and Leah?
Malbim: They gave Birkas Chasanim. There were three Brachos. (a) They blessed her - even though she was a Nochri's daughter, because she came to the house of a Tzadik like you, her virtue should be like Rachel and Leah. Also they were daughters of Lavan ha'Arami, but because they married Yakov, they built Beis Yisrael. So will be built from her a Bayis Ne'eman - a house of kingship on which the success of all Yisrael depends (Mashi'ach comes from this house)! Also refer to 4:11:3:1, 4:12:1:2.
Malbim (17): The Ari Zal (Sefer ha'Gilgulim) says that oppressed Neshamos nurture influence via their oppressors. They trick their oppressors and show to them a blemished place; they think that the Neshamah will remain in their hands. In the end, the Neshamah overpowers its oppressors. The Neshamah of the Shevatim was put in Rachel and Leah. They were married b'Isur; [the oppressors] thought that due to this, they will remain in Tum'ah. The same applies to Peretz' Neshamah in Tamar, and the Neshamah of Na'ami and Rus in Mo'av. Therefore, David said "Bnei Ish Ad Meh Chevodi li'Chlimah" (Tehilim 4:3). Rus Rabah 8:1 says, they said that he comes from Rus ha'Mo'aviyah [so he is forbidden]. David replied, "Imru vi'Lvavchem Al Mishkavchem" (ibid., 5) - did you not come from the Isur of [marrying] two sisters, and matters like this!? The Am and Zekenim sensed this, and said "she should be
Why did they mention Rachel before Leah? They were from Shevet Yehudah, who is from Leah!
Rashi: They admitted that Rachel was Yakov's primary wife.
Why did they bless "va'Ase Chayil b'Efrasah"?
Malbim: This was a Brachah to Bo'az. The name of his city is Beis Lechem; his family was called Efras, for they had good lineage. Ase Chayil includes all perfections, to the point that they will be glory for the entire family of Efras. Refer to 4:11:4:2.
What is the meaning of "u'Kra Shem [b'Veis Lechem]"?
Rashi: Your name should increase.
Malbim: Via fame of your name, the name of your city will be famed for praise, that someone like you was born there.
It says (Yevamos 77a) that Do'eg said that David is Pasul, for even a Mo'avis is forbidden. How could he say that Bo'az (the Shofet Ivtzan) transgressed? Also, all the Am and Zekenim gave to him a Brachah for marrying Rus!
Maharsha (Yevamos 77a): Do'eg said that the Mitzvah of Yibum overrides the Isur and permits one Bi'ah. Rus became pregnant, and Bo'az died, and never did a second Bi'ah. Yibum used to apply also with other relatives, like the Ramban said 1 .
Tzitz Eliezer (17:42): Also in Bo'az' time, Tov and others 2 dissented; the argument persisted to the days of Sha'ul. Bo'az died the night after he had Bi'ah with Rus. This supported the dissenters!
Ramban (Bereishis 38:8) said so about before Matan Torah. However, at Matan Torah, Yibum with other relatives was forbidden! Tzitz Eliezer (17:42) - how could a custom (Yibum with other relatives) override a Torah Lav?!
The verse says that "Kol ha'Am
