From Adobe Campaign, how is a segment defined in another Adobe Experience Cloud solution accessed?

A-Bar Enterprise; has a marketing workflow which incorporates Push notifications. User IDs are now changing to the hashed values of the previously used ID. Each recipient already has this unique hashed value stored in a field in the Recipient table.
Mobile app subscriptions are still registering correctly in Adobe Campaign, but the workflow no longer knows which recipient is linked to each User ID.
Assuming everything outside of Adobe Campaign has been configured correctly, what must the developer do to regime tending Push notification?

Which campaign variable gives the results of a target data count?

A client would like to send multiple recurring email deliveries using the same Sender address. What would a developer use to store the Sender address to efficiently manage changes across all existing deliveries and environments?

A client is using Their own Customer schema instead of the default Recipient schema. What out-of-the-box functionality do they lose if they do so?