What are the potential foreseeable financial, psychological, and medical, yet unintended, harmful consequences to one’s family and friends in failing to provide a properly executed will and living will prior to one’s final illness and death?

The main thing is everything will be tied up in probate for a long time.  The court will decide who gets what.  So if you wanted your grandmother's special ring to go to your first born granddaughter and that is not written out, it might not happen that way when the court decides things.

Life insurance money will be tied up as well so funeral expenses and other bills that were intended to be paid will that will go unpaid.  If you were intending your family to live off your life insurance for a time or pay off the home so they would not have to worry about a house payment, this could really put them under stress because they won't have that income while that money is in probate.

Without a living will the family may not know what your wishes are and even if you have told them in the past, without them in writing if you are unable to speak they cannot legally carry them out.  If your wishes were to have a Do Not Resuscitate order but you did not complete one, the doctors have to care for you including putting you on machines.  This may be against what you and your family wanted.  

It is hard enough watching a loved one get terminally ill and then die.  But without the legal documents that make that passing at least a little more bearable, the financial and emotional stress to the family can be a huge burden that could be prevented.