There seems to be a variance between answers I'm getting from the Nexus and on Steam, so I would like to bring this question to home here at these forums.

I know people say "corpses are supposed to disappear after three game days" etc...

Is this fact or a generalization? Will certain corpses ever disappear (Forsworn "buddies" I killed at their base after I helped them escape, dragons, Umbra 3.0 - who by the way WON'T STOP SHOWING UP in my game)?

What about items? How long before a dropped item or some skeleton's bits-and-pieces disappear?

What about crash landing damage done by dragons? Will this pox on the beautiful terrain remain forever?

If they are set to disappear, will entering the cell or nearby cells reset their clocks?

Corpses will usually disappear unless they have a persistent/essential item in their inventory. I think there is an exception for Ulfric Stormcloak however. Essential items are usually limited to those pertaining to quests. When these "Buddies" you speak of respawn, their earlier corpses are no longer in the game.

I'm not sure about the rest, though I think that crash landing damage does reset. Call it 90%.

If you keep returning to the same cell within that 72 hours, the 72 hour countdown resets each time.
Meaning that you need to not visit that cell for three full days, or the corpses will never disappear because you keep telling the game to remember them by being near them.

When I am done playing for the day, I will go to a small interior (like an inn or my house) and rest 72 hours all at once, and then save my game.
It clears away junk the game was holding onto for no reason.

720 hours (ten days) for locations not marked as cleared, and 30 days for cleared locations.

Actor held in aliases or carrying items held in aliases will never disappear.

Some NPCs and/or creatures may have scripts attached to them that do not shut down themselves properly. Such NPCs, when killed, will NEVER disappear from the game without 'help'. With Skyrim being as buggy as it is, this happens a lot more often than some might think.

An excellent example would be a small group of bandits (three, I think) near Orotheim. They have a script that has them running across the stream to the entrance of Orotheim when you first visit the area, and not much else. These three bodies (assuming you kill them) will never disappear. This hasn't been patched, either.

There are several methods to combat this. In my game, the combination of official and unofficial patches removes the ash piles very efficiently immediately upon leaving the cell, so all I have to do is reanimate them and when they disintegrate they are gone without a trace upon exit/entrance.

The same method works with so-called 'permacorpses' the game has placed on purpose, like the dead khajit traders in Eastmarch etc. etc. (there are hundreds of those).

Whenever I grow tired of them, I dispose of them permanently by the same method (just make sure you remove any irreplaceable assets they may have, like treasure maps, skill books, and the like).