Extensibility Options in the context of a SAP BTP, ABAP Environment Multitenant Application?

Currently I am evaluting the usage of SAP BTP, ABAP Environment for a Multitenant Application scenario. One evaluation point is what extensibility options that scenario will provide. As I did not find any documentation regarding that, I want to address my questions here and maybe Florian Wahl or Andre Fischer or someone else can deliver valuable input for that topic.

By the way, I saw that in the Roadmap Explorer for Q3 2021 improvements for extensibility in multitenant scenarios are planned, but the given information does not clarify any questions.

Let us assume we have following scenario:

• A multitenant application on SAP BTP, ABAP Environment; RAP and Fiori Elements are used
• Consumer A wants to extend a BO with additional properties, expose that via the existing services and include that in the existing Fiori Elements application (to make it a little bit more complicated in a extension section which displays a freestyle implemented fragment).
• Consumer B wants to extend a BO with additional properties too. In addition he wants to add additional actions.

Questions:

• RAP specific: BOs can be extended with extension view entities, but how the behavior and service layer can be extended to add e.g. additional entities, actions, functions?
• Are such extension scenarios supported in case the tenant-mode for the ABAP solution is "multi"? "multi" means to my understanding that the data "isolation" for different consumers is done by client separation on the same shared ABAP environment system. So I would assume that e.g. extending the same BO with additional properties from different consumers is not supported.
• Are such extension scenarios supported in case the tenant-mode of the ABAP solution is "single"? As in single mode for each consumer a separate ABAP Environment is provisioned and access to such systems is possible with development tools (according to documentation) I guess that in this setup extensions are possible. Event the deployment of additional artifacts. Of course a setup is necessary which provides a separate "development", "test" provider infrastructure which allows the consumer to develop and test on separate instances, before moving the solution to the production subscription system.
  One drawback of the "single" tenant-mode approach is of course the additional price tag, because each single provisioned system needs to be payed.
• Will "classic" key user extensibility (including UI Flexibility) be supported on "multi" provisioned ABAP solutions?
• Will UI Adaptation projects be supported on "single"/"multi" provisioned ABAP solutions in future (at the moment the documentation says no)?

Questions (non-related to extensibility):

• One additional question not related to extensibility: In the Landscape Portal an overview about the system landscape is given with options to update the created ABAP AddOn. In case of a "single" tenant-mode, does it mean, that the AddOn update can/must be done for each single provisioned system separate or is there an option to update all systems to the same AddOn version at once.

Thanks for your feedback.

Florian