“大势一去水东流” seemingly means "大势一去 就如長江水东流" (when defeat becomes inevitable, it is like the water of Yangtze River flow East, --It can't be turned back)
Using "River water run East" metaphorically for the meaning of "irreversible situation" is very common in Chinese poetry, the following are some examples:
《念奴娇 赤壁怀古》宋·苏轼:"大江东去 ....." (The water of Yangtze River flow East and won't return...)
~
《臨江仙》楊慎:
"滚滚长江东逝水...." (The water of the rolling Yangtze River disappear to the East...)
~
《虞美人》 南唐后主李煜:
"....一江春水向東流" (....The water of a river flow East in Spring)
“事其所以观其所由, came from 论语 “视其所以,观其所由,察其所安,人焉廋哉!"
大势一去 is referring to the idiom 大势已去 宋·朱熹《朱子谏类》卷五十一:“.... 莫是大势已去。”
The phrase 水东流 appeared in the first line of 满江红·"汉水东流...."
The '戏'(opera) that 古老夫子 '唱'(sung) was a mix and match from different sources -- an original creation from the author of 《普提山下》陈果卿