Yes. The SA can be used across multiple offers as long as these are under the same Publisher Account. Using the SA across different publisher accounts will be blocked by the certification automation at stage 1 of publishing.
Yes. The SA's config page in the admin site (e.g. XYZ-admin.azurewebsite.net) allows partners to upload their logos. After a restart of the WebApp, the new logo will be seen on the top right of both the billing landing page and the admin page.
Yes. The implementation of custom meters can be done separate from the SA installation. After the marketplace subscription is active, parter can emit meters using their own/custom metering service. The client ID and secret for the SA can also be reused by the custom metering service to connect to the metering API.