I don't know what the terms of your status challenge were, but normally one gets choice benefits each time one qualifies for a status level (PM or DM). You have until the end of your status benefits period to make your selections, which is January 31st in the normal case. I don't understand how you have PM choice benefits for both years, but the answer might lie in the T&C of your status match.

Getting DM means an additional set of three choice benefits, but won't eliminate the PM choice benefits, whether already selected or not.

RUCs come from different upgrade inventory ("buckets") until close to departure (although I'm not sure whether it's technically day or departure, T-24, or gate control) than free elite upgrades. Then all remaining seats become available for upgrade, usually at the gate, and RUCs are prioritized ahead of all requests for a free elite upgrade.

If your RUC requested upgrade doesn't clear, the RUC becomes available in your account for application to another flight, up to its expiration (one year after being selected). This should be automatic, but if not, you should call DL.