Vilna Gaon #1: Pe'ulah is every act by itself. Schar is like human kingship. If one does a service many times, in addition to paying for each, the king gives another reward for all the duration of his service, according to his generosity and wealth. All the more so Divine kingship will do so! Bo'az blessed Rus that Hashem will do so to her, like He does to Yisrael, who serve Him from birth. The reward for each act is in this world, and reward for the duration of his service is in the world to come - there is no Schar Mitzvah in this world! Even though she was a Nochris for her first years, He should not diminish her reward.

Vilna Gaon #2: Our Avodas Hashem is not in order to receive reward. How can an Eved Olam request reward? His master totally owns him! Hashem took us out of Egypt to be His slaves. One who was not redeemed from Egypt, e.g. Rus, can request reward. He or she chose [to serve] Hashem!

Vilna Gaon #3: There is no Schar Mitzvah in this world, for the reward of a Mitzvah is a Mitzvah ¹ , i.e. that Hashem merited us to do [His will]. They told me of your good deeds - do not expect reward in this world, rather, He will pay you [in the world to come]. You should be pleased that all your reward in this world will be "that you came to take shelter under His wings" - the reward of a Mitzvah is a Mitzvah.

Malbim #1: "Pa'alech" corresponds to what you did for your mother-in-law. "Maskurtech" for converting should be complete from Hashem, for He is Elokei Yisrael and oversees them and converts who join to them. A Midrash says, a Po'el receives wages for the work that he did, e.g. a tailor who made a garment. A Sachir is hired for a fixed time, e.g. three years, whether or not he works. One is rewarded for each Mitzvah like a Po'el. Refer to 2:12:2:2.

Malbim #2: This is because he acquires perfection of the Nefesh, or for being a servant of the great King, the Creator. Why did the Torah promise reward for Mitzvos? The Mitzvah itself is the greatest reward! Indeed, for one who understands the pleasantness of Mitzvos, there is no need to promise any other reward. Most do not appreciate this; they needed promises of reward in this world, so they will do the Mitzvah amidst hope for reward, until they reach the level of doing Mitzvos Lishmah. Hashem should pay the reward of your actions, but the primary reward is "that you came to take shelter under His wings."

Refer to 2:12:1:3, 2:12:1:5.

Malbim citing a Midrash: A Po'el receives wages for the work that he did, e.g. a tailor who made a garment. A Sachir is hired for a fixed time, e.g. three years, whether or not he works. One is rewarded for each Mitzvah like a Po'el. For coming to convert, you accepted to serve Hashem and fulfill all His Mitzvos, like one hired for his entire life. He deserves constant wages, whether or not he works, i.e. whether or not he had opportunity to do a Mitzvah.