What is the LDS line of authority should a Bishop be unavailable? I am seeking a complete and authoritative explanation of this issue.

Who presides at Sacrament meeting (common Sunday services) when the bishop of a ward (pastor of a local congregation) is unavailable?  There are several circumstances that should be considered when answering this question.  The best answer will cite both the handbook of instructions and scripture to justify the explanation.  Linking back to Church resources is appreciated.


When any higher authority is easily accessible and...


When the Bishop (perhaps due to illness) is suddenly unavailable.
When the entire bishopric is unavailable, but the absence was expected.
When the entire bishopric is unavailable, but the absence was unexpected.

When all higher authorities are also unavailable and...


When the Bishop (perhaps due to illness) is suddenly unavailable.
When the entire bishopric is unavailable, but the absence was expected.
When the entire bishopric is unavailable, but the absence was unexpected.



A bonus would be an explanation of what priesthood keys (or priesthood authority) is NOT available when any or all of these circumstances take place.  As an example (and as a hint), if the bishop and all higher authorities are unavailable, non-priesthood disciplinary councils cannot be convened.
The policy for who presides at sacrament meeting is found in Handbook 2, 18.2*:


  The bishop oversees ward meetings. He presides at these meetings
  unless a member of the stake presidency, an Area Seventy, or a General
  Authority attends. His counselors may conduct ward meetings and may
  preside if he is absent. Presiding authorities and visiting high
  councilors should be invited to sit on the stand. High councilors do
  not preside when attending ward meetings.
  
  If the bishop and his counselors are all absent, the stake president
  designates who presides at sacrament meeting. Normally he designates
  the elders quorum president, but he could authorize another
  priesthood holder instead.


So, if the bishop is absent, his counselors preside. In practice, this usually means the first counselor, and if he is absent, it is the second counselor. If neither the bishopric nor his counselors are present, whoever the stake president designates will preside.

If the entire bishopric is unexpectedly gone and the stake presidency is unreachable to designate who will preside, then circumstances are extraordinary and chances are that a normal sacrament meeting would not be held.

The sacrament can only be administered with authorization from the bishop (who holds priesthood keys for ordinances like baptism and the sacrament within his ward boundaries), or the stake president, who holds similar keys within his stake. The bishop may authorize a priesthood holder to administer the sacrament in his ward boundaries, outside of a formal sacrament meeting. Nobody presides per se at these informal sacrament services, but the person who the bishop designates conducts (see 18.2.2 in the Handbook chapter linked above).



*Prior to April 2019, when this answer was originally written, the quote from Handbook 2 referenced the "high priests group leader" instead of the elders quorum president. The high priests group leader position no longer exists as of April 2018, so the quote has been updated to reflect the newer wording in Handbook 2.