“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...)
"滚滚长江东逝水...." (The water of the rolling Yangtze River disappear to the East...)
"....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