1. If you are referring to the kind of code the I/O assistant will generate, it will generate VISA code, where the case you may have an option is for a GPIB resource (in which case you can generate code which implements the communication using the GPIB/NI-488.2 API).
2. Error Code BFFF0015 means "Timeout expired before operation completed."
3. You can use the VISA API and simply specify an RS-232 port to communicate with for the resource name. I have attached a program which will show you how to do a loopback test (write to and read from the same serial port; connect/wire pins 2 and 3, and then run the attached program).
4. I am not sure what you mean by "configure the USB setting using LV" but if you are new to USB and LabVIEW the following two links will be great starting points:
a. What is USBTMC and how can I Communicate to my USB Instrument Using NI-VISA? http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/044fa220f32774ed86256db3005850ca?OpenDocument
b. How Can I Communicate With a Device Using NI-VISA USB RAW Mode? http://digital.ni.com/public.nsf/websearch/E3A2C4FE42D7ED0D86256DB7005C65C9?OpenDocument
Hope this helps you get a good start using VISA and LabVIEW! Repost if you have further questions!