Then you're breaking the law. You're in charge of the vehicle and a lot can happen whilst you're sitting at traffic lights in terms of other vehicles and people moving around you. If the lights change and then you just drop the phone and move off, in all likelihood you'll do so without making proper checks. People really need to get it into their thick heads that you shouldn't use phones whilst driving and that includes being at traffic lights. The majority of accidents happen at junctions, try concentrating when you're operating dangerous machinery.
That's a pretty big leap. You can move off without proper checks even if you were starting at the red light for the last ten minutes. You can also be found to be breaking the law for tuning your radio, or eating a single gummy bear, which are no more distracting than changing gear.
Most accidents happen at junctions, but they involve moving or stopping cars most of the time, not people moving off without checking.
I'm pretty sure my six month old is more distracting than my phone, but I'm allowed her in the car even when it's moving!
I think most people I know probably break multiple laws a day operating on common sense, because the law is a blunt instrument and humans are not robots.
All that said, how exactly does scaring the shit out of the poor woman make the situation any safer? She's unlikely to become hyper vigilant at junctions unless it's to look out for aggressive cyclists.