You would think that people would have enough common sense, but on Monday when I rode Splash Mountain a kid stood up every few minutes simply to try to see what was ahead. I am not going to speculate on the motives of guests who prematurely exit the ride vehicle, but if Disney doesn't have any preventative measures in place to prevent that, they are liable. And announcements and signs are not legally sufficient.
Disney can't be liable for the stupidity of those who get out of a ride vehicle between boarding it and stepping off at the end of the ride. Going by your logic then most rides on Disney property are unsafe - a couple other MK examples would be IASW and Tomorrowland Raceway (as the seatbelt can easily be undone) - whats to stop guests getting out of those ride vehicles? The same will apply for most rides at parks worldwide. Disney would surely only be liable in the event of a ride malfunction that resulted in guest injury due to inproper maintenance / failure to comply with health and safety regulation related to theme park ride operation / negligence by cast members etc.
If a ride is operating - such as splash is at the moment - incompliance with regulations, and a guest voluntarily gets out of the ride vehicle whilst in ride, whether it be stopped or in motion - how can this be the fault of Disney?
Also, if announcements and signs are not legally sufficient then why can Disney, and various other theme park operators worldwide operate like this?
I think you'll find that rides will be subject to various health and safety checks before they commence operations, and regualrly inspected to ensure they comply with local and national regulations and that the rides and shows in the parks will be operating in accordance to these. Perhaps you could direct your issues at those who regulate such health and safety / ride operation laws as opposed to Disney and the cast members who will be working, as the Disney Company is, in accordance with these regulations.
Also, I would imagine that in terms of how a ride can be restarted the processes to go thru this will be restricted to the ride design and technology of the time, hence why they may seem 'overcomplicated'. Depending on the ride infrastructure this would merit significant down time - which would not be required if the ride is still safe and compliant with current regulations. If these regulations changed, resulting in a redesign of some elements of the ride, then I'm pretty sure you'll find Disney would take the ride down to implement them.
And one more thing - the positioning of cast members throughout the ride as you suggested, explain to me how this will stop guests from getting out? Surely if a CM is on one side of the ride, then a guest could still easily get out of the ride vehicle, onto the side of the CM or more easily the other side. In addition, i do think that it would be money wasted positioning CM's throughout the ride for the relatively few times the ride will stop for a prolonged period of time - especially as annoucements can do the job, as well as a CM's sent thru the ride to see everyone if needed. The ride i assume, like most, will be covered by CCTV and if there was an issue in a particular area a CM could be dispatched there.
This is probably why on the Tomorrowland Raceway there is a CM on the bridge to cover the blackspot in that area. As far as the reassurance side of things goes, people of a nervous disposition will still be nervous no matter what a CM says and no matter how many there are, especially if hearing various alarms, sirens, and announcements for the benefit of CMs located throughout the attraction. You could actually go too far in the other direction - if you had that many CM's stationed throughout then guests may get the impression the ride is unsafe that its staffed this much. Its the same with nervous flyers - the engines may rev, or the flaps may come down, the plane may drop slightly - all perfectly normal conditions of a flight - and a flight attendant can reassure that person all they want, but they will still be nervous, want to undo their seat belt etc. You cant make everything 100% safe. You cant wrap everyone in cotton wool. Thats just the way life is.