I have no experience with the CDC but I have some with the MSD class. Some steps I did and may be useful for you. First, let's create a stand alone project:
1) I assume you have downloaded and installed the MAL (Microchip Application Libraries). Copy to your example directory the following files: usb.h, usb_ch9.h, usb_common.h, usb_config.h, usb_device.h, usb_device_local.h, usb_hal.h, USB PIC18.h, usb_function_cdc.h, usb_function_cdc.c and add them to the MPLAB X project. Remove the libraries from the MPLAB project.
2) Compile it, link it, and see if it's still working ok.
3) Using MPLAB X, remove all the code lines that are grayed in the #ifdef or #else preprocessor blocks. That is, remove all the code intended for other architectures that are not intended for a PIC18F45XX.
4) Repeat 2.
5) If you start studying the code now, you'll notice that it's much simpler than what you thought in the beginning, and you will have much more control over you project.
Hope this helps.
If your device shows up as a CP2013 when you plug the device into a computer's USB port, and you want to replace your Computer with an Arduino, then that isn't really easy to do, or generally possible. The methods you mention in the question have to do where the AVR/Arduino is the device, not the Host.
The device already has a USB-UART converter embedded inside it, and only shows you the USB side of it. This means that your arduino needs to function as a USB host. Functioning as USB host is not straightforward. You need to set up a USB Host/Hub controller, and figure out how to handle enough of the USB protocol stack to talk to the CDC compatible USB-UART device. Using a USB OTG Host controller may be marginally easier, but still nowhere close to being trivially easy. Your best bet would be to see if you can bypass the CP2013 on the device and access the UART lines directly. This may not always be possible. If the CP2013 is actually some other piece of hardware or firmware masquerading as a CP2013, then you're fresh out of luck.
Best Answer
It is not a straight forward usb-cdc/virtual com port. The Scale uses a USB-HID slave/peripheral controller. This needs to be read using USB-HID. It is completely different from USB-CDC, a.k.a. a usb virtual com port. It does not use a USB to RS232/TTL adapter.
See http://steventsnyder.com/reading-a-dymo-usb-scale-using-python/ and https://stackoverflow.com/questions/1275974/program-for-hid-usb-scale and https://www.youtube.com/watch?v=srvtDNYR05U
Also, your idea of using a USB to Serial cable will not work that way. A USB to RS232 cable is not bi-directional. It can not be used in reverse. It will not connect to a USB peripheral/slave. It has to be connected to a USB host/master, like a computer.