Hey Serena,
I think we have two point of confusion here. First, we are talking about three different endpoints.
1) /canned_questions
2) /questions
3) /questions/:id
Second, questions and canned_questions are completely separate things.
To get canned_questions, you must use the canned_questions endpoint. The canned_questions endpoint does not accept and :id as a GET parameter.
You want to get a canned_question by id where the id is "tax_registration_id" and unfortunately you cannot do this.
You are correct that there is an endpoint that let's you get questions by id and that the id of the tax id canned question is "tax_registration_id".
I do apologize for the confusion. We can certainly do a better job of documenting how these things work and that is something that we are actively working on.
When it comes to the responses of the questions, you are also correct. We are not providing that data. I just realized that myself. And I apologize for the misleading post that I left earlier indicating that you would be able to get the responses with the canned_questions endpoint.
I can add this to our backlog of API feature requests for you but I can't speak to how long it will take for the change to take place.