1)
Why does it say "Al Hashem"?
1.
Radak: This is like El. Similarly, "va'Tispalel Al Hashem" (Shmuel I, 1:10) is like El Hashem.
2)
Why does it say both "Leshareso" and "Lihyos Lo la'Avadim"?
1.
Malbim: Leshareso is light service - Avodah and Mitzvos. Amidst love, they will ascend to the level of Avadim who serve constantly, day and night. One who serves amidst fear does only what is required and needed.
3)
Why does it specify Shabbos?
1.
Refer to 56:2:5:1-3.
