As far as we are concerned, artillery doesn't feature in the rules and therefore the answer to most of these questions is "whatever you think is appropriate to your scenario". In armies like the Romans, artillery would be manned by legionaries and so would have the same cohesion as them. In other circumstances, go for C6 unless there is a strong rationale for it to be higher. I suggest that you do not allow artillery to be moved, you are just making things more complicated for yourself.
"No one ever achieved anything without making a few mistakes along the way"