Can sendtoaddress use account other than ""? sendtoaddress is using the "" account by default. Is it possible to make it use any other account name?
Accounts are deprecated and labels are the alternative for accounts.
I'd recommend using latest Bitcoin 0.17.0 which has multi wallet features which can isolate each user, further read:

https://en.bitcoin.it/wiki/API_reference_(JSON-RPC)#Multi-wallet_RPC_calls
https://github.com/bitcoin/bitcoin/pull/13058