Any help with this would be great.
If the vehicle behind you couldn't brake in time to avoid an accident with you in front, i'm not sure where your charge comes from. If anything, he should be charged.
Hand/Arm signals are sufficient for farming equipment as long as you are plainly visible. I'm not sure what the issue with your left hand turn is here. Maybe he is claiming you were sitting in the shoulder when you made your turn? I am not sure.
I'd suggest you request a trial and request disclosure as soon as your Notice of Trial comes in the mail. This way you can see what the reasoning is behind your ticket. Doesn't sound like they would have a case convincing anyone this is deserving of a careless charge, if any charge at all.