Either getting turned let him pick new spells as if he had a full rest, or he had left a few low level slots empty that day and filled them during the time the Order was stuck in the illusion.
Lying not necessary.
Possible. Detect Magic is 0th level, but you can put lower level spells in higher level slots, and a new bonus 1st level slot would do the trick.
Edit - And Rich answered already. Whoops.