3. How can I cite Octave?

Pointing to http://www.octave.org is good, because that gives people a direct way to find out more. If citation of a URL is not allowed by a publisher, or if you also want to point to a traditional reference, then you can cite the Octave manual:

@BOOK{eaton:2008,
  author =     "John W. Eaton and David Bateman and Søren Hauberg",
  title =      "GNU Octave Manual Version 3",
  publisher =  "Network Theory Limited",
  year =       "2008",
  isbn =       "0-9546120-6-X"
}