What Happens To The Extras?

The Mint can only sell commems in the year of issue. I really doubt they plan to rewrite that policy. I have no clue what they do with specific leftover coins but with alloy items I would assume they would be destroyed.

The Mint does produce its commemorative coins in batches based on its initial and ongoing sales projections for each issue. This is done to limit the costs associated with the production/packaging/warehousing of coins that may, ultimately, not be sold. When you see a coin's status listed as "Back Order" on the US Mint web site, it means that current inventory is depleted and that another batch is coming but not yet struck or fully packaged and ready for distribution.

The authorizing legislation for commemorative coins includes a date range for the period the coins can be sold by the Secretary of the Treasury (effectively, the US Mint). While it is possible that Congress and the President could authorize and approve, respectively, a new future sales period for a coin, such an amendment would be unlikely. For one thing, it would create a situation where previous commemorative coins would be competing with current coins which would create an issue with the sponsor(s) of the current coins (and this would go against current law).

Unsold precious metal coins (silver, gold, platinum) are melted, not "stockpiled" for future sale. The Mint now uses a process nicknamed "waffling" to cancel / void its non-precious metal coins. The process is generally used for circulation coins that are defective in some way, but it would serve as well for remainder collector coins that can't be sold. Once waffled, the pieces are no longer considered coins - just scrap metal - and are sent off to recyclers. I haven't seen a waffled clad commemorative half dollar as of yet, so I can't say for certain that the process is being used for them - they might be melted.