Guild Missions being Cancelled?
Nope. It’s one thing that had been asked for Anet to provide numerous times.
It’s a great feature that anyone can start a mission and, in theory, you want anyone who can start it up to also have cancellation privileges. In one of my guilds, this allows everyone to work on missions even if they aren’t available for the official guild mission times. In practice, that works because the guild is very selective about new members — there’s a high level of trust to begin with.
In many guilds, especially those that recruit PUGs regularly, it makes sense to limit who can start|stop missions — the guild won’t know yet who can and should be trusted with special powers. For the same reason, this is why some parts of the bank will be restricted and why not everyone will have privileges for dropping banners or capping wvw objectives, etc.
That said, it would be nice if the guild /log specified whether a mission failed or was cancelled (and by whom), among other changes.
Sorry for the late reply, Thank you for the info!