No one else can modify the owner's role. Storage for the group is drawn against their quota. Only the owner can delete the group or transfer ownership to another member.
fcheslack answered the question about privileges -- the three things listed are the three differences between admins and owners; there are no others.
what do you mean by "storage for the group is drawn against their quota"?
Attachment storage in groups is counted against the Zotero File Storage quota of the group owner, so if the group has >300MB (or all libraries of the owner have >300MB) they'll need a paid account for attachments to sync (metadata will continue to sync either way). Any storage (paid or unpaid) by other group members is both irrelevant and unaffected by groups they don't own.
Beyond that, might be helpful to describe a specific situation you're looking at rather than asking general questions where it's hard to tell what answer exactly you're looking for.
Upgrade Storage
Storage for the group is drawn against their quota.
Only the owner can delete the group or transfer ownership to another member.
Beyond that, might be helpful to describe a specific situation you're looking at rather than asking general questions where it's hard to tell what answer exactly you're looking for.