Please be careful entering the XPath equations, particularly note the single quotes ' vs double-quotes ", the distinction is critical otherwise the formula wont parse correctly, and you'll get errors like what you are seeing. I tried these (in Kobo) just now and they do appear to work as desired, so please re-check your form.
Correct, in Kobo, relevant is the 'Skip Logic' stuff. Unfortunately, Kobo doesn't expose the full required functionality; it only allows you to specify true() or false(), via the "Mandatory?" check box. So if you are using Kobo to write your form you will have to do some further post-processing to put that into your XForm before uploading it.