What is the purpose of the two metaphors Kennedy uses
in the phrase: "If a beachhead of cooperation may push
back the jungle of suspicion..."

no its not a run on sentence however i think you meant wet suits not sours also a run on sentence would be like surfers surf all winter long in southern california by using wet sours to keep them warm and the live their all year long  

the characters in string all have major secrets that they strive to avoid revealing throughout the play.

  this had halp

comfort   in their suffering