Did you sync all data to multiple computers without a Zotero account? Zotero's documentation said WebDAV could sync attachment files. But everything else required a Zotero account.[1]
No you can't. I have both a Zotero account and a WebDAV server. Since bulk of the load is the attachments, doing it this way doesn't bother me, actually.
[1] https://www.zotero.org/support/sync