Are there any outstanding critical security issues for Sql Server 2000?
Yes, there are no patches available for newly discovered issues like meltdown/spectre
Microsoft only releases security patches for SQL versions that are still supported. Also notice that if you are running a supported SQL version with an unsupported service pack, you MUST upgrade to a ...
You can use sp_dropalias do drop the legacy SQL Server 2000 user alias in SQL Server 2008 R2:
EXEC sp_dropalias N'<name-of-aliased-login-here>';
This stored procedure was removed in SQL Server 2012 later versions.
You didn't say what backups you have. If you only have a full backup, then there's no point-i-time restore. It can only be restored to the "snapshot in time" it reflects (towards the end of the backup period).
If you do have also transaction log backups, then I agree that you should test SSMS and see what RESTORE commands it generates (you have a script ...