Backup internals - What happens when a backup job is running - in terms of locking and performance overhead in SQL Server?
Your all points are covered in backup myths - by Paul Randal

30-01) backup operations cause blocking

No. Backup operations do not take locks on user objects. Backups do cause a really heavy read load on the I/O subsystem so it might look like the workload is being blocked, but it isn't really. It's just being slowed down. There's a special case where a backup that has to pick up bulk-logged extents will take a file lock which could block a checkpoint operation – but DML is never blocked.


  Also I heard that backing up is single threaded meaning it uses only one core, assuming you backup to a single file.


A backup when done to a single file or device will use 1 writer thread. So if you are backing up to Multiple files /devices (be that multiple .bak files) will have one writer thread per file/device.


  The easiest way to improve backup performance is to allow the backup operation to parallelize, which is known as backup striping. By default, there’s a single data reader thread for each drive letter or mount point being read from and a single data writer thread for each backup device being written to.


Check 


SQL Server 2008 Microsoft Certified Master (MCM) Readiness videos especially Backup Internals.
A Look at Backup Internals and How to Track Backup and Restore Throughput (Part 1) - By: Jonathan Kehayias
A Look at Backup Internals and How to Track Backup and Restore Throughput (Part 2)- By: Jonathan Kehayias