One or two slash depends on where the file is, it should have a certain answer for that, no need to try.
In my opinion, the path starts with two slash is used to access to remote directory in network, and the service pub.file:getFile could be used only for local, so you should always pass the path starts with one slash to this service.
So the question is, is that really a local file? Because there is a folder named b00 under root really seems weird.
Could you run this command successfully from shell?
cat /b00/inbound/InterfaceName/Monitoring/abc.xml
#webMethods#Integration-Server-and-ESB#Flow-and-Java-services