Description of problem: Currently, specifying either --noformat or --useexisting results in both --noformat and --useexisting being passed as parameters which prevents their use for different purposes. The intent is to use (with anaconda) --noformat on a subvol command to indicate just adding the entry to /etc/fstab and using --useexisting to cause the subvol to be destroyed and then (re) created [the equivalent of a "reformat"]. It would be nice if this was in Fedora 20 but I see that as hopeless.
For btrfs, the --useexisting option is the same as --noformat. https://pykickstart.readthedocs.io/en/latest/kickstart-docs.html#chapter-2-kickstart-commands-in-fedora I don't know enough about btrfs to even know if it's possible to do what you're describing. Even if it is, pykickstart is not the right component to report the bug. pykickstart just parses the kickstart file and anaconda uses it. Anaconda would pass on storage work to blivet, but the right place to ask would be anaconda. btrfs support is marked as deprecated in pykickstart, so it might be a good time to move off btrfs anyway.