For your first question, based from the Migrate to Google Drive API v3 documentation:
Full resources are no longer returned by default. Use the fields query
parameter to request specific fields to be returned. If left
unspecified only a subset of commonly used fields are returned.
Your code should be:
service.about().get().setFields("user, storageQuota").execute()
The fields query parameter should be specified for methods which
return
For your second question, as far as I know, even if Drive storage is shared between Google Drive, Gmail and Google Photos, your service account will not use the drive storage quota of your main account. You can read more from this documentation:
Service accounts should only be used for performing delegation where
the effective identity is that of an individual user in a domain.
Using the service account as a common owner to create many shared
documents can have severe performance implications. Additionally,
service accounts may not acquire additional storage quota, nor do they
act as members of a domain.
You can share your account's folder to your service account to get the additional storage from your normal account.