Q1: An opponent's Digimon without the highest DP attacks, and by using a When Attacking effect to increase its DP, it becomes the Digimon with the highest DP. In such cases, can the target of the attack be switched to this Digimon with this card's Opponent's Turn effect?
No, you can't. If Digimon did not have the highest DP when the attack was declared, this card's Opponent's Turn effect can't be activated.
Q2: Does this card's "When a Digimon becomes unsuspended" effect activate even when my opponent's Digimon becomes unsuspended?
Yes, it activates.
Lv.6 BlackWarGreymon (X Antibody) BT11-074 SR Mega | Virus | Dragonkin/X Antibody Digivolve: 2 from [BlackWarGreymon]<Reboot>[Opponent's Turn] [Once Per Turn] When your opponent's Digimon with the highest DP attacks, you may switch the target of attack to this Digimon.[Opponent's Turn] [Once Per Turn] When a Digimon becomes unsuspended, if this Digimon has [BlackWarGreymon] or [X Antibody] in its digivolution cards, you may delete 1 of your opponent's Digimon with the lowest play cost.
Lv.6 BlackWarGreymon (X Antibody) BT11-074 SR Mega | Virus | Dragonkin/X Antibody Digivolve: 2 from [BlackWarGreymon] <Reboot> [Opponent's Turn] [Once Per Turn] When your opponent's Digimon with the highest DP attacks, you may switch the target of attack to this Digimon. [Opponent's Turn] [Once Per Turn] When a Digimon becomes unsuspended, if this Digimon has [BlackWarGreymon] or [X Antibody] in its digivolution cards, you may delete 1 of your opponent's Digimon with the lowest play cost.
Lv.6 BlackWarGreymon (X Antibody) BT11-074 SR mega | Virus | Dragonkin/X Antibody [Digivolve] [BlackWarGreymon]: Cost 2 <Reboot> [Opponent's Turn] [Once Per Turn] When your opponent's Digimon with the highest DP attacks, you may switch the target of attack to this Digimon. [Opponent's Turn] [Once Per Turn] When a Digimon becomes unsuspended, if this Digimon has [BlackWarGreymon] or [X Antibody] in its digivolution cards, you may delete 1 of your opponent's Digimon with the lowest play cost.
Lv.6 ブラックウォーグレイモンX抗体 BT11-074 SR 究極体 | ウィルス種 | 竜人型/X抗体 「ブラックウォーグレイモン」から2≪再起動≫ 【相手のターン】[ターンに1回]最もDPの高い相手のデジモンがアタックしたとき、アタックの対象をこのデジモンに変更できる。 【相手のターン】[ターンに1回]デジモンがアクティブになったとき、このデジモンの進化元に「ブラックウォーグレイモン」か「X抗体」があるなら、最も登場コストの低い相手のデジモン1体を消滅できる。