Jailbreaking does nothing but install
one file, an additional developer certificate, allowing update.bin files signed by the MobileRead Kindle Developers' tools, to authorize them so the Kindle will run them. Those updates however, will typically modify all sorts of things.
Expired library loans are treated as regular books, except that Amazon's servers will silently delete them, and report a message that the book is a loan and has expired, if you try to download them again. Without WiFi/3G, it will simply tell you to connect, leading me to believe the info is not stored locally. They will also send a personal document "letter" to notify you that the loan has expired. All this I know from day-to-day use, so there may be other traces left in the filesystem, I do not know.
I know the Kindle stores the WiFi info, since it remembers networks and automatically connects (with passwords saved) but I have no idea where.