What happens to the lifespan not collected by Shinigami?
This is actually covered by the Rules of the Death Note:


  How to Use: LIX
  
  A human death caused by Death Note will indirectly lengthen some other
  human's original life even without a specific intention to lengthen a
  particular person's original life span in the human world.


It is likely that this simply means that the person killed "prematurely" has extended other people's lives as they go about their lives differently than if the person was there, thus events leading to their deaths may change.

No other rules of the Death Note cover what you are looking for. Glancing at a few likely places in the manga and reading over the wiki, I cannot find anything specifically stating if anything happens.

Due to Misa's death shortly after the series, we can assume that she gained no extra life span from her efforts. Even when she reduced her life span to a quarter by trading for the shinigami eyes twice, she should have still lived a long time if killing someone influenced her life since it would have been added afterwards.

Additionally, we know that the shinigami owner of the death note does not gain the life span because they did not write the name themselves.

Based on this, I would say that the left-over time is wasted. Other people's lives will be influenced due to the rule above, but the "remaining" time is not distributed in any means.