A) We only see Redcloak casting prepared (memorized or on scrolls) spells on the go.
B) You mean he wants to hide a "a few hour-gap" from the Xykon that had arrived just minutes ago (enough to make Jirix get blue, but not enough to make him die). We're looking at a max of 10 minutes. How could Redcloak have brought the thing to Xykon earlier when Xykon wasn't even in town (or on the plane).
I'm seeing no reason at all to kill the Spy but to be "really cautious". Note I understand Redcloak taking that approach but we have not seen any better reason so far.