The phrase "keep nothing back from you" is an idiom that means to tell someone everything. This can be stated in positive form. AT: "I will tell you everything the Lord tells me" (See: [[rc://en/ta/man/translate/figs-idiom]] and [[rc://en/ta/man/translate/figs-activepassive]])
Here these words share similar meanings. They describe Yahweh as a witness whom no one will be able to contradict. AT: "trustworthy" (See: [[rc://en/ta/man/translate/figs-doublet]])
The people mention both extremes to emphasize that they will obey no matter what Yahweh's answer is. AT: "Whatever he answers" (See: [[rc://en/ta/man/translate/figs-merism]])
Here "voice" represents what Yahweh says. AT: "what Yahweh our God says" or "we will obey Yahweh our God" (See: [[rc://en/ta/man/translate/figs-metonymy]])