If a credit memo generates a revenue arrangement with recognition in closed accounting periods, should I expect the resultant JE's have a transaction date in the closed period as long as the "Posting Period" is set to the earliest open period? How should I be interpreting that?

yes, it it will post to the first open period for all prior periods and then will continue posting an “offset” to the rest of the periods. this is because you have your start and end date in each revenue arrangement - original and cr memo