I recommend asking the Old Town Moms for the going rate for nanny shares in the neighborhood. Plenty of families will reply directly to you - I've heard $17-20/hr but that was a couple years ago. 2 weeks vacation, 1 week sick is typical. For coverage, most families sort it out independently rather than trying to hire a joint back-up nanny. You might plan your vacation to coincide with the nanny's while the other family will fly in grandparents.
In terms of hours, you'll probably need to have agreed set hours for the share at the share rate. If one of the families is late for pick up, it's a bit complicated because if you're hosting and the other family is late, then the nanny is sticking around your house with some one else's kid and not on the clock for you. Your kid will probably still want to play with his friend in the play room, not entertain himself in the kitchen while you make dinner. Alternatively, you could agree to let the nanny go on time and then you're stuck babysitting the share's kid. It's probably best that the general plan is that everyone is on time for pick up.
To your other questions, there would be set core hours, ie M-F 8am -6pm. When one family goes on vacation, they still pay. If one family picks up early, they still pay. If one family is running late, that family is 100% responsible for any additional OT owed to the nanny. You could set up complex systems to cover all the possible scenarios but it's complicated and adds an additional layer of rules & systems to an already complex relationship.