The WMF core features team tracks this bug on Mingle card https://mingle.corp.wikimedia.org/projects/flow/cards/562, but people from the community are welcome to contribute here and in Gerrit.
Lack of cancel button is intended, as it doesn't make sense in no-JS context.
Without JS, you're taken to a designated page to reply/edit/... You can just hit the browser 'back' button to go back to where you came from. There's no special kind of interaction going on that has to be cancelled.
Preview is something else. That too is intentionally lacking, but for other reasons: it's a lot of work to build a no-JS alternative for every feature, so some (non-crucial to discussion) features are ignored (for now).