But you should be able to slow down or reverse your route with the right stick. But that won't react quick enough most likely to stop any unplanned obstacles strikes. My advice is first flight of new CCC plot you want the have the telemetry visible and be ready to exit per the UAV toolbox warning.
Over on this side of the pond it's the left stick ;~)
Andy's right: the UAV Toolbox missions perform exactly the same functions as on-the-fly-created missions. And, bumping the stick up or down to slow/reverse direction works surprisingly quickly. I've had a couple poorly planned missions that prove the point.
You can "stop" and hover in between way points with left stick input. I just ran a mission I'm using to learn UAV toolbox and you can dynamically control speed with gentle forward or backwards nudges on the left stick.