Hi Brenden,
1. You can use both DI API and also Service Layer in order to work with your UDOs, UDTs and UDFs in your B1 version for SAP HANA. Service Layer metadata includes the required objects to create the UDOs, UDTs, UDFs (UserObjectsMD, UserTablesMD,...) and also the entities required to work with the data.
2. You can keep using UI API and develop the backend part with Service Layer (then replace the DI API with Service Layer).
SSO between UI API and Service Layer is available since B1H 9.2 PL01.
Regards,
Trinidad.