I own this one.
The question was about providing onward travel tickets instead of return tickets.
I looked at the 'canonical' and noted that it could be improved by changing 'return' to 'return/transit'. I made the change. Then I hammered the OP's question as a duplicate.
We then spent about 45 minutes in chat where I explained how to create a new question about VFS and gave the OP some ideas about why it was important to separate the visa code from VFS from solo females etc.
I also explained that he can lodge a complaint in META.
What is the policy on editing the accepted answer to an older question
and then marking a new question as duplicate?
There is no policy. The effect is that you got the answer to your question.
But if someone can edit an answer on an old broader question to
include the answer to your new specific question, then all questions
on the site are potential duplicates?
Changing "Return tickets" to "Return/Transit tickets" is a permissible edit and helps answer more questions. This same thing has happened to nearly all the canonical questions on the site (see 'do I need a UK transit visa' for example).
Check the edit history on this canonical.
The newer question is specific while the 'Canonical' question is too
broad and still does not answer it entirely. Thoughts?
Yes, you broadened your question and rescoped it to the extent that it's now 'too broad'. Now it includes solo females, VFS, border controls, and more.
Marking a question as duplicate is also a rap on the knuckles that
says "you did not search well enough".
Marking a duplicate means your answer is someplace else. Feeling a rap on the knuckles is natural, but cannot be avoided. I wonder if your reaction is proportionate to the situation.
Denouement: your question has been reopened. I would not be surprised if the community now marked it as 'too broad', but all the best of luck in getting a satisfactory answer.
Epilogue: the question was closed by the community as 'too broad' thus acquiring the rare distinction of a single question being opened, closed, reopened, and reclosed all within a 12 hour period. None of the valuable suggestions and help offered by the community have been implemented as of yet.