The etymology of 便当 in Chinese The following excerpt is from the top answer on 知乎:


  丈夫每天吃自己做的便当（没错，是他自己做的，不是他妻子做的）她吃大餐，去一次美容院就能花4.2万。看表情就知道，这位太太毫无愧疚之心，可能还觉得自己萌萌哒。两人摊牌后，妻子没有道歉，而是要求丈夫以后更努力工作赚钱。最后以这男人跳下站台自杀，妻子改嫁结束。


The 便当 here means a bento-box. However, since it is clear that "bento" in English is a loanword from Japanese, I feel that it is a loanword from Japanese in Mandarin as well, not a word that had been incorporated into Japanese.

However, the Japanese word is 弁当, not 便当. So my question is:


Is my assumption correct that it is a loanword from Japanese?
If that is true, then why is it called 便当 in Chinese? And when did it start to be called as such?
I try to make a rough translation here:


  便当， originated from the Southern Song Dynasty, meaning "convenient
  things, convenient, smooth". 
  
  When introduced into Japan, it became “便道”、“辨道”、“辨当”. 
  
  Later, when the term “便当” was transplanted back to China from Japan,
  it's referenced to 弁当(bentou/bento). 
  
  The term 盒饭, (that is 盒装餐食), is used in mainland, while
  便当 is usually used in Taiwan. 
  
  However, there is a nuance between 盒饭 and 便当. 便当 is more delicate than
  盒饭.  盒饭 is usually just simple and normal food.