Hi,
1. No, you need to enter each value in the custom field. But it should only take 10 minutes.
2. I'm not following you. If the rental times are 6 hours and that you already have a start time for the customer to enter, where do you want to "populate 6 hours" ? There is no need for an end time selection if the duration is fixed to 6 hours.
And if you want to populate such field to be selected start time + 6 hours, it will require adding custom javascript code to the page.
So if you're a developer, it shouldn't be a problem.