Glad you feel a bit calmer- not that I blame you for being angry and upset.
I think you have every right to say that you would prefer EXH to do long distance driving, rather than grandparents. However, there is a good chance that this will upset them all and to insist on it may look obstructive. I wouldn't worry about upsetting EX, I think that is inevitable unless you just roll over and give in to everything. However, upsetting the grandparents maybe not a good idea and make things more difficult for you in the future in terms of 'rubbing along' together. No doubt they think they are perfectly capable of driving DCs, and may take it very personally that you consider them too old / unhealthy / reckless to do so. I casual "drive carefully won't you" while looking into grandpa's eyes is fine though!!
It would be really good to find out how the whole set up for the court thing may work. I am sure its not all like you see on TV. Maybe CAB could help, they do free sessions. Or maybe someone on MN can advise. I am sure preparation is important and will help you to feel in control but at the end of the day (sorry to repeat myself), they are dealing in facts, most of which you will already know. You know how you feel about the situation, you know the history, you know your girls.......
I wonder what type of preperation you could do? You are keeping all correspondence and a record of anything significant (no matter how small, how often he speaks to DCs- everything), file this in a way that if required you could access things easily and quickly. You could try writing down and practising what you would like to say- you will already know what you need to say but may need to practise a little to keep calm and concise and show that its the DCs interests you are protecting, you are not trying to punish XH etc.
You could also have a few key phrases to keep yourself focussed on sounding as reasonable and rational as possible. Am not suggesting you are not reasonable or rational at all but guess in the heat of the moment it could be tempting to get personal with EXH.
I have totally jumped the gun here but just had a few thoughts- hope you don't mind.