Do the effects of skills casted upon enemies overridden or overlapped?

take Necromancer’s skills as examples, if I cast Decrepify first then Iron Maiden, do they overlap or being overridden by the last one?

Only one curse can be active on a target at any time. Shrines count as curses for some reason.

Usually only one Curse can affect a mob at a time. Most Curses dont stack, even when using multiple necros.
I believe if the same curse is applied, the stronger of the two wins out.

For paladin auras you can have a many stacked, again the same auras only apply the higher level one

This is true. Another set of abilities that don’t stack are Fade and Burst of Speed.