Who does "Bnei Yosef" refer to?
Rashi: It is Shevet Menasheh. 1
Bi'ur ha'Gra, Malbim: It was Efrayim and Menasheh.
How does Rashi explain verse 17? Yehoshua told Beis Yosef, i.e. Efrayim and Menasheh, "if you are populous
What was their complaint?
Radak: Eretz Yisrael was apportioned to those who left Egypt, and Menasheh had only 32,200 [men above 20] then, and [in the latter census in Parshas Pinchas] they had 52,700. (On average, each man received about three fifths of a portion.) Efrayim would not complain; their population decreased from the earlier census. Also, Eretz Yisrael was divided into 12 equal portions; a big Shevet received the same a small Shevet. 1
Bi'ur ha'Gra: Seven 2 equal portions were entered in the lottery. After the lottery, Yehoshua needed to add to bigger Shevatim from adjacent Shevatim. He did not do so for Bnei Yosef.
Malbim: Efrayim and Menasheh received only one portion 3 , just it was split into two parts.
Menasheh had about 2,600 more than an average Shevet. Yehudah should have complained. They had over 26,000 more than average! (PF)
Below (18:4,5), Sheluchim made seven portions among the rest of the land. Yehudah and Yosef already received beforehand, via lottery (15:1, 16:1). Perhaps the Vilna Gaon mentions the seven to teach that Yehoshua added to other Shevatim, but not to Bnei Yosef. (PF)
How can he say so? Bamidbar 34:14,23,24 says that nine and a half Shevatim will inherit in Eretz Yisrael proper, and different Nesi'im will inherit (acquire a share via the lottery) for Efrayim and for Menasheh! Perhaps Menasheh's Nasi acquired their joint share (refer to 17:1:1:3, and Efrayim's Nasi acquired Efrayim's share amidst it. We can say similarly about Shimon acquiring amidst Yehudah (19:1). Above (14:4) it says that Menasheh and Efrayim are considered two Shevatim; Malbim must explain that after receiving one portion, it was split into two. (PF)
Why did they say "Ad Ko Berchani"?
Rashi #1: They increased from the first census 20,500
Did Bnei Yosef get anything due to their complaint?
Radak: They did not; Yehoshua could not change inheritances. However, he counseled them to dispossess the forest nearby. The mountain was of Efrayim, and the valley of Menasheh, and from there they could spread to Eretz Perizi and Eretz Refa'im nearby.
Malbim (17,18): Yehoshua was authorized to adjust the size of the portions according to the population. 1 He now added to them from Asher and Yisachar. 2 Those Shevatim would not mind, since they themselves could not dispossess the Kena'anim there, who were strong.
Malbim (15): Yehoshua could give to them only in their Techum, but not from elsewhere. (Below (verses 17,18), he says that Yehoshua was authorized to adjust the portions based on the populations of the Shevatim! Perhaps here Yehoshua thought that they request another Techum because they should be considered like two Shevatim. Below, he agreed to help them due to their population.
