For the duplicate id: +1, I like it.

For the extensions to conditional vars: it's a good idea in theory, but I'm less sure it's useful in practice.

An example error that this branch flags:

mandatory: True
code: |
    ...
    if has_evidence:
       review_exhibits
     ocr_task  
    ...
---
question: |
  You are required to submit copies of relevant Trial Court documents. 
fields:
  - Do you have any documents to share with the judge?: has_evidence
    datatype: yesnoradio
  - Choose all of the documents you want to upload: exhibits
    datatype: files
    show if: has_evidence
    ....
---
id: review exhibits
question: |
  Review your exhibits
subquestion: |
  Look at the exhibits below. If you want to change the order, use the arrows
  to move an exhibit up or down in the list. Click "delete" or "add another"
  to change which exhibits you send to the court.
  
  ${ exhibits.table }
  
  ${exhibits.add_action() } 

There are no errors with this code, and it's fairly standard for conditional questions to do things like this. It is assumed that review_exhibits is connected to exhibits existing as an object, sure, but I can't think of a more clear or concise way of writing this code. Idk.

Added @nonprofittechy as a reviewer, I feel like he'll have some good thoughts on if this is useful for folks or not.

See comment--can use the duplicate ID check, and an unconditional reference check in attachments, but should be conditioned on not using skip undefined. when skip undefined is turned on, no check in attachments is necessary.