If the Khol Wall's text allowing its Provinces to attach two regions is nullified (e.g., Amnesia, etc.), the regions stay where they are, and are not discarded.
Although the Diamond rulebook implies that legality of regions are checked continuously (i.e., "Each Province can only have one region"; Diamond rules, pg. 59), the situation falls under the rule on pg. 55, Diamond rules: "The conditions for playing or attaching a card matter only while a player plays or attaches it."
This has been confirmed by Jeff Alexander via private email, and a correct Archive entry will be entered later today. I'll fix my posts, too. (Actually, found I had ruled correctly the first time this question got asked in the forum!)