Orange -- "can they [ manglement ] make you work more than 12 hours?
-- Basically - yes. You are expected to deliver ALL of the assigned mail each day. As one unpopular member of manglement liked to remind us carriers "It is what it is." Of course manglement does not like carriers saying that back to them during the Christmas No-Overtime Period!
-- If it takes 12 hours or more to deliver your route, so be it. While manglement has to pay for anything over 12 hours, anything above the under-evaluated route's time to the 12 hour mark is free work for the USPS. ( something the NRLCA seems to overlook at contract time - IMHO ).
-- Manglement is not supposed to schedule RCA's for more than 12 hours of evaluated work -- however, they are relatively unconcerned if an RCA takes more than 12 hours to complete the day's evaluated work. Again, paying for a few minutes of overtime work past 12 hours is, no doubt, worth it.
"can they make you come in at 6am rather than 8am with less than 24 hours notice?"
-- Yes -- While there should be "reasonable advance notice" made of scheduling changes, most carriers would jump at the chance to come in early on expected heavy days, such as the day after a holiday. ( You could try grieving it, but I'd imagine other carriers would try to talk you out of it. You would have to show how you would be "harmed" by the change in schedule - such as the inability to find a baby sitter on such short notice, or the day care doesn't open that early, etc. But remember the USPS does not care that you have a family to care for. Something like the military: If they wanted you to have a family, one would have been issued in your duffel bag or in the case of the USPS, at indoctrination. )
-- Section 332.34 of the M-38 ( Management of Rural Delivery Services ): Carriers may be scheduled to report earlier than normal [ a temporary change ] on days when the preferential mail volume is greater than normal, if it is necessary to maintain the regularly scheduled leaving time. However unless operational needs dictate, starting times should not be prior to 0600 ( 6 am ).