Q1: I am in a situation where there is an opponent's Digimon with DP 8000 or less. Can I not select the Digimon to be deleted by this card's When Digivolving effect?
No, you cannot. If you have an opponent Digimon that you can choose, you must choose that Digimon.
Q2: I am in a situation where I have an opponent's "BT14-062 Datamon" and an opponent's Digimon with DP 8000 or less. Can I select "BT14-062 Datamon" with this card's On Digivolving effect and fulfill the "If this effect does not delete it" requirement without making the Digimon delete?
Yes, it is possible. You can satisfy the "if not deleted by this effect" condition by choosing an opponent's Digimon that is not deleted by this effect.
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。