Rulings
Q1: This card's When Attacking effect refers to "your opponent's Digimon with the highest DP." Does this strictly refer to my opponent's Digimon with the highest DP, or does it refer to when I attack an opponent's Digimon with the highest DP out of all Digimon in play, including my own?
It refers strictly to your opponent's Digimon with the highest DP they control. The effect doesn't check the DP of your Digimon.
Q2: My opponent has multiple Digimon tied for highest DP. If I attack any one of them, does this card's When Attacking effect let me unsuspend this Digimon?
Yes. If your opponent has multiple Digimon tied for highest DP, attacking any of them allows you to unsuspend this Digimon.
Q3: My opponent's Digimon with the highest DP is unsuspended. If I attack their unsuspended Digimon with the highest DP, can I unsuspend this Digimon?
No, this Digimon's When Attacking effect only lets you unsuspend this Digimon when attacking your opponent's Digimon with the highest DP, regardless of whether it's suspended or not.
Data Sources
Lv.6 BlackWarGreymon BT2-112 SEC Mega | Virus | Dragonkin While your opponent has a Digimon with 10000 DP or more in play, reduce the memory cost to play this card from your hand by 6.[When Attacking] If you attack your opponent's Digimon with the highest DP, unsuspend this Digimon.
Lv.6 BlackWarGreymon BT2-112 SEC Mega | Virus | Dragonkin While your opponent has a Digimon with 10000 DP or more in play, reduce the memory cost to play this card from your hand by 6.[When Attacking] When this card is attacked by your opponent's Digimon with the highest DP, unsuspend this Digimon.
Lv.6 BlackWarGreymon BT2-112 SEC Mega | Virus | Dragonkin When you would play this card from your hand, if your opponent has a Digimon with 10000 DP or more in play, reduce the memory cost by 6. [When Attacking] When you attack your opponent's Digimon with the highest DP, unsuspend this Digimon.
Lv.6 ブラックウォーグレイモン BT2-112 SEC 究極体 | ウィルス種 | 竜人型 DP10000以上の相手のデジモンがいる間、手札のこのカードを登場させるとき、支払う登場コストを-6する。【アタック時】最もDPの高い相手のデジモンにアタックしたとき、このデジモンをアクティブにする。