NFS access isn't restricted by account based ACL, but using it's own access rules instead. The NFS access rules works independently from the POSIX layer ACL and the Windows ACL. 
Under the share configuration page, firstly select to enable the NFS share and click apply, then re-enter the same page to find the NFS access rules section and add the rules accordingly.
The root squash prevents the NFS client access from mapping a root account UID to the storage's root account, if the root squash is selected and the NFS clients uses a root account UID for mapping to the NFS share, the access credential will be automatically mapped to "nobody". Select this option to avoid security threads from unwanted root access.
Using Sync access rule would caused the NFS access performance to drop down to 10%~30% of Async access in average. So it is recommended to use Async access whenever possible unless you strongly demend the safely in data transaction and have no or less concern about the data access performance. 
Yes, U1/U2/U3/U4/U6 all support multiple nodes. The number of nodes is 64.
The Windows ACL is currently only configurable directly from Windows clients. Once enabled (in the share configuration page, select "enable ACL"), to make Windows ACL functions properly you'll have to set the POSIX layer ACL to "all pass" condition, which means setting the POSIX layer ACL to all RW permissions, and let Windows ACL work as the only ACL to filter the access from clients.
Currently CIFS/AFP/FTP use the same POSIX layer ACL and cannot be configured separately. WebDAV uses its own ACL set and NFS uses its own access rules, both independent from the CIFS/AFP/FTP ACL. 
For all series QSAN NAS, the maximum number of LUNs that the user can create are 256. 
Yes, users can attach multiple LUNs to the same volume. But we will suggest users to attach multiple LUNs in the cluster environment or in share storage environment. If attaching multiple LUNs to serveral servers without the cluster/sahre storage environment, the data would be corrupted. 
No, all NAS series don't support iSCSI LUN masking feature due to patent. 
Yes, user can backup/restore the user account in WebUI. Please go to the configuration page, selecting the Account in the left part, then the user can export/import account via this page.