A 207 is stored but not complete. Some answers failed the form's rules; they are kept, marked invalid, and the message reads "The Form Submission has been saved successfully, but it contains validation errors." Nothing that counts or acts on completed submissions sees it, so a client that treats every 2xx as success thanks the visitor for a submission nobody will read.
Show the errors from validationErrors, keyed by element name, and keep the submission's token from the answer. When the visitor corrects them, send every answer again to PUT /cdna-api/forms/submission/{token}, which completes the same submission: a new POST would start a second one. Treat 200 as done and 202 as "go and pay".