Should I let my ex go on a long car trip and help in an emergency, or remove myself from the situation entirely?

WOW! What a predicament! I suspect your fears are well-grounded. Even before Mild Cognitive Impairment (MCI) develops into Alzheimer's disease, the ability to drive can begin to be impaired. It is not about his knowledge of practical driving lessons such as stopping at red lights or being in the right lane to make a turn, but it is about reaction time, and detours, and processing information, and making decisions, and unfamiliar places, and reading signs, and the occasional need for multi-tasking while driving, etc. All these are affected by MCI. As his physician suggests, he may be "ok to drive" but I'm fairly sure his doctor refers to around town, short distances, and on familiar routes. I guess the question is, "Is 'OK' good enough for a long distance drive"? I think not! How lucky he is to have your support. I'm sure you don't want to interfere with this opportunity for him to have family-time but perhaps his cognitive deterioration (MCI) should take precedence over a family get-together.
You are correct about having 'no legal right' to intervene but perhaps there is a moral obligation to let his daughter know your feelings and try to dissuade her from insisting he do the driving. Four hundred miles is a long trip for anyone without cognitive issues - how about a nice train ride? Do suggest to his daughter that, even if she is not in accord with your thoughts, she does need to have a backup plan and to be prepared to quickly change plans. I suggest this to all families in similar situations. The best you can do is give his family the appropriate information/recommendations and pray you never have to say, "I told you so!" Be well.