

Q1: My opponent has a Digimon with 8000 DP or less. Do I have to choose a Digimon to delete for this card's When Digivolving effect?
Yes, you do. If your opponent has a Digimon that can be chosen, you must choose that Digimon.
Q2: My opponent has BT14-062 Datamon and a Digimon with 8000 DP or less. If I use this card's When Digivolving effect to choose BT14-062 Datamon without deleting a Digimon, is the "if this effect didn't delete" condition met?
Yes, it is. If your opponent has a Digimon that can't be deleted but can be chosen for this effect, the "if this effect didn't delete" condition can be met by choosing that Digimon.
Lv.6 Gallantmon BT19-015 R Mega | Virus | Holy Warrior/Royal Knight [When Digivolving] Delete 1 of your opponent's Digimon with 8000 DP or less. If this effect didn't delete, until the end of your opponent's turn, this Digimon gains <Piercing> and gets +3000 DP. [Your Turn] [Once Per Turn] When any of your opponent's Digimon are deleted, gain 2 memory. ---
Lv.6 Gallantmon BT19-015 R Mega | Virus | Holy Warrior/Royal Knight [When Digivolving] Delete 1 of your opponent's Digimon with 8000 DP or less. If this effect didn't delete, until the end of your opponent's turn, this Digimon gains <Piercing> and gets +3000 DP. [Your Turn] [Once Per Turn] When any of your opponent's Digimon are deleted, gain 2 memory. --- -
Lv.6 Gallantmon BT19-015 R mega | Virus | Holy Warrior/Royal Knight [When Digivolving] Delete 1 of your opponent's Digimon with 8000 DP or less. If this effect didn't delete, this Digimon gains <Piercing> and gets +3000 DP until the end of your opponent's turn. [Your Turn] [Once Per Turn] When an opponent's Digimon is deleted, gain 2 memory.
Lv.6 デュークモン BT19-015 R 究極体 | ウィルス種 | 聖騎士型/ロイヤルナイツ 【進化時】DP8000以下の相手のデジモン1体を消滅させる。この効果で消滅しなかったなら、相手のターン終了まで、このデジモンは≪貫通≫を得て、DP+3000。 【自分のターン】[ターンに1回]相手のデジモンが消滅したとき、メモリー+2。 ---