So you want to both argue that they could not understand the mindset of one of the Scribblers they did not share the mindset of, but could understand the mindset of another of the Scribblers they did not share the mindset of? That's certainly a bold strategy, I'll give you that.
The runes were purely magical - Reddie and Xykon are equipped to understand pure magic. Neither of them have many, if any, demonstrated skillpoints in Medicine or Knowledge (Nature), however, so it is pretty unlikely they would figure out why Xykon lost his powers, especially since Redcloak didn't, without being explicitly told why.
Let's take a Lawful view - would Redcloak accept the terms presented to him immediately by a Devil, without suspecting something? Heck, Redcloak didn't even accepting the terms of a Lawful dwarf without suspecting something a few strips ago.
This is not a strictly Lawful issue.
Not at all. I'm saying that powerful, free-willed undead, like Great Wyrm dragons, are rare in this setting. I believe I have been very clear on this point.
You mean the Gate that Redcloak and Xykon wouldn't even have known was there if some other goblin hadn't told his god?
Imean, that was your argument against Tarkin's army, right? He didn't find it himself so it doesn't count?