Q1: What exactly does the digivolution card's inherited effect mean "while memory is 0 or less"?

This digivolution card has the digivolved card's digivolution card effect, which means that if the Digimon with this card as its digivolved card has a memory of either "0 to 10 on the opponent's side" at the time it activates the DP Deletion effect, the condition has been met.

Q2: What does "DP deletion effect limit +X000" mean?

This effect adds the upper limit of the value texted in the DP deletion effect. For example, "On Opponent's Turn Delete one opponent's Digimon with DP3000 or less When Digivolving" while "On Digivolving Delete one opponent's Digimon with DP3000 or less" is activated. will allow you to delete an opponent's Digimon with a DP of 4000 or less.

Q3: While the "DP Deletion Effect Limit + X000" effect is active, can I add an upper limit to the effect that deletes opponents' Digimon with DP less than or equal to this Digimon's DP, such as "Deletes opponents' Digimon with DP less than or equal to this Digimon's DP."? Can I add an upper limit to the effect that deletes the Digimon by referring to its DP, such as "This Digimon's DP is less than or equal to the opponent's Digimon's DP"?

No, it cannot be added. You cannot add the upper limit of the DP deletion effect for which no numerical value is texted.