1. Is Saturday overtime paid at other branches ? I’d heard it was.
2. 1st class mail volume is down due to the internet (email & the web). End junk mail and the total volume drops even after the recovery.
3. Every-other-day mail delivery would only be for mail recipients. Half the delivery routes would occur on one day, the other half on the next.
4. USPS mail delivery personnel wouldn’t have a longer route, they would just have two routes, alternating every day. Their route size each day wouldn’t change.
With 2, 3 & 4 above, only half the work force would be required.
Then, there's the problem of "number of stops" ~ let's say a route had 500 delivery points or possible deliveries. On an average day you'd probably have mail going to at least 400 of them. The time it takes the carrier to cover that route is highly dependent on actual delivery stops ~ because it takes a certain amount of time to actually deliver mail at a stop (Stop. Get mail from tray. Open window ~ in winter. Reach out to box. Open box. Put mail in box. Close box. Check for traffic. Travel to next box.
It gets far more complex with foot routes, park and loop, etc.
When you cut delivery to every other day on any particular route you increase the number of actual stops ~ probably all the way up to 500 every time you deliver.
Apartment routes have other problems ~ and so on and so forth. USPS has numerous employees dedicated to making sure these routes can be delivered efficiently. Every idea you presented will result in greater street time requirements. You'd have to ADJUST all of the routes as a result, making them shorter. Then you'd need more routes established, with more carriers.
I'm sure your aggregate savings would be about 30% of what you imagine is possible.
That's just on street time ~ which is ideally more than half a carrier's time, but not all of it. There's office time and that's a totally different problem. All the things a carrier does in the office before going out are dependent on the volume of mail, not the length of the route.