How did they return to test Hashem?
Radak: After they requested meat, and they knew that they sinned, via Hashem's anger that flared at them, they tested Him at Mei Merivah and other matters not mentioned in the Torah. It says Kel, for He is over everything, and they thought that He lacks ability, and they tested if He can do this and that.
Malbim: They tested when He can supply all their needs that they asked each time.
Why is He called "Kedosh Yisrael"?
Radak: He was sanctified in Yisrael via many wonders that He did in front of their eyes.
What is "Hisvu"?
Rashi: It is an expression of a Siman, like "v'Hisvisa Tav" (Yechezkel 9:4). They requested a wonder [to show] "ha'Yesh Hashem b'Kirbeinu Im Ayin" (Shemos 17:7).
Radak: They limited Him. They put a border in their hearts - this He can do, but this He cannot do. He can change nature only until this point. This is like "v'Hisvisa Tav" (Yechezkel 9:4) and "Tesa'u Lachem" (Bamidbar 34:7); it is a border and sign of a matter.
Malbim: They put a border around Him to measure His strength, ability, desire
