I'm sorry, but this is quite simple- either your mother is truly a danger to herself/others or she is not. Your father should not have required any prepping if he was telling the psychiatrist the truth and you should not be involved if he was not.
Being hospitalized against your will is a violation of sorts and should only be done to protect the patient or others from physical danger. Please think about the actual impact it will have on your mother to be treated this way.
Why don't you visit your parents for awhile and then speak to a professional only about the behaviors you actually witness, see if they can give you advice on how to approach your mom to speak to someone. Maybe the approach is what is off, maybe your mother just doesn't respond to "You've got a problem. What is wrong with you?" followed by wayward Disneyland trips that end up at the hospital- genuine loving concern might work better.
P.S.- Maybe you shouldn't worry about your dad so much, if he is willing to divorce his wife of many years while he believes she is suffering from an untreated mental illness- he doesn't need your worry - he is thinking about himself more than enough...IMHO
Blessings