1)
Why does it mention both "Yanum" and "Yishan"?
1.
Radak: Tenumah is less than Shenah. "Lo Yanum" and all the more so "Lo Yishan." He is "Shomer Yisrael" every moment!
2.
Malbim: He does not doze due to fatigue and weakness, and the disposition to sleep does not apply to Him at all. A person, even if he need not doze due to tiredness, he needs someone to guard him when he sleeps.
2)
Why is Hashem called "Shomer Yisrael"?
1.
Radak: He guards us even when we sleep, and even in Galus, lest the Goyim eradicate us. They desire to do so, for they hate us.
