An ASPSP is free to use a proprietary interface. He is obligated to provide the corresponding information and documents to TPPs. This includes in particular a specification of the implemented interface and a testing facility. In case of reasonable doubts that the used interfaces is compliant to the XS2A specification, a TPP can contact the National Competent Authority.
An ASPSP has to provide a testing facility beyond September 14th 2019. So, again, it is highly recommended to a TPP to test a new service prior to taking it to the productive system of the ASPSP. This ensures the TPP is granted technical support of the ASPSP in case of issues. Extending the services may require to renew the TPP’s certificate and license such that the TPP is, in fact, permitted to provide those services.
It is referring to different possible account identifiers (IBAN, BBAN but also aliases like MSISDN).
The ASPSP will choose the account type for the implementation of its /accounts endpoint.