The maximum number of shares that user can create is 512.
Yes, users can enable multiple share types in the same file system. Such as, CIFS, NFS, FTP, AFP. 
No, only users can access to his own folder. (There is no path for admin to access other userhome folders.)
Compared to other commonly used LAN file sharing protocol such as CIFS and AFP, WebDAV can be used over Internet. And compared to commonly used Internet file sharing protocol such as FTP and Http, WebDAV has the advantage of providing file-locking mechanism, as well as on-line editing possibility.
No, the WebDAV owns it's own ACL set, you may configure it separately in the WebDAV settings page.
From the Web UI, the configurable ACL permissions on the share configuration page refers to POSIX layer ACL. And the Windows ACL works as an additional set of ACL attached to the folders and files.