my old JD baler has and overrunning clutch built in, pretty sure all of them do. You would break shear pins every time you stopped the PTO if you didn't.
Lost to rot really depends--how wet (and wicking) is the ground, how much rain at a time, how much sun to dry it down, condition of the hay going in. A good stack sheds
water like a thatch roof. I think you lose less in a stack than in round bales out in the weather. Part of that is the size--the manual stacks we used to make were 10-12 TONS each so they had a lot of volume for the surface area. A good stack might have a foot of degradation in the top and 6" in the sides, but a bad spot on the top could funnel water all the way through in a big column. They are just so time intensive, especially the moving them after you make them.
My Grandpa had a neighbor that stacked manually (well, a cable loader on a farmall M) long after everyone else had switched to round bales. But he left the stacks in the field and winter-grazed the field. He put up electric to keep the
cattle away for the other stacks and put a feeder around the one stack in use. Rotational grazing long before anyone knew what to call it.
Best video I have seen is the bit in the BBC's wartime farm. I can't remember the episode. They put it 12" up in the air on
mushroom shaped stones to keep the
mice and rats out of it plus airflow in the wet British weather.
Making most of the stack is just like building a square
compost pile--pile it neat and straight edges (except DRY). The only difference is making the dome at and laying the hay to shed water on top.
I have done it with scythe and pitchfork, but I cheated and threw it on my trailer and used the truck to haul it home. I just did 20-25 cu yards of it this weekend for making compost. I cheated and used my loader in the end.
Keep reminding me on the ripper, it isn't done yet. I have more parts, and a plan, but garden production took priority over pasture work this spring. The cattle will be rotated through the area I was going to rip in another month, so it may be tight getting the keylines in before the fall rains start--but who knows what the weather will be this year, it has been anything but normal so far! Plus I need a new phone and/or camera.