In their rebellion. Several MSS. have b'Mitzraim for b'Miryam, which would give the sense "appointed a captain to return to their bondage in Egypt." So the Septuagint. Appointed a captain. The reference is to Numbers 14:4, where we are told that the Israelites "said one to another, Let us make a captain, and let us return into Egypt."
The Levites speak as if the appointment had been made, perhaps regarding the intention as morally equivalent to the act. A God ready to pardon. Literally, "a God of pardons." The word used is a rare one, occurring only in Daniel 9:9 and Psalms 130:4, besides the present passage.