.. and we've yet not even touched on the possibilities of undocumented isolated cases of mercy killings of our own soldiers;
and desperate measures under extreme duress to silence loud hysterical panicking young soldiers...???

You will find such incidents referred to in many accounts including the memoirs of Sassoon and Graves who were both very critical of the war.
Graves also speaks of prisoners being murdered and it being treated as a joke.

Summary executions I have never come across in any accounts.