Such effects change the original DP shown on the target card to the predetermined value shown in the effect. For example, if a Digimon has an original DP of 8000 and it's chosen as the target for a "[Main] Change the original DP of 1 of your opponent's Digimon to 3000 until the end of their turn" effect, that Digimon's original DP will become 3000 instead of 8000.

Its original DP is changed, then +1000 is added to the changed DP. Values added to DP don't affect the original DP.

The original DP is changed to the most recent value.

No, it can't. Effects that change information can't change information that a card didn't originally have. For example, if a Digimon has its original DP changed by an effect and then its top card becomes a card without DP due to a <De-Digivolve> effect, it will no longer be affected by the effect that changes the original DP.