Q1: My opponent has a Digimon with 4000 DP or less. Can I meet the "if this effect didn't delete" condition in this card's On Play When Digivolving effect by not choosing a card to delete?
No, you can't. If your opponent has a Digimon with 4000 DP or less, you must choose that card and delete it.
Q2: My opponent has a Digimon with 4000 DP or less. Can I meet the "if this effect didn't delete" condition in this card's On Play When Digivolving effect by choosing an opponent's Digimon with 4000 DP or less and an effect that prevents deletion?
Yes, you can.
Lv.4 Growlmon BT21-068 C Champion | Virus | Dark Dragon [Digivolve] Lv.3 w/[Guilmon] in name: Cost 2 [On Play] [When Digivolving] Delete 1 of your opponent's Digimon with 4000 DP or less. If this effect didn't delete, trash the top 2 cards of your deck. --- Inherited: [On Deletion] Gain 1 memory.
Lv.4 Growlmon BT21-068 C Champion | Virus | Dark Dragon [Digivolve] Lv.3 w/[Guilmon] in name: Cost 2 [On Play] [When Digivolving] Delete 1 of your opponent's Digimon with 4000 DP or less. If this effect didn't delete, trash the top 2 cards of your deck. --- Inherited: [On Deletion] Gain 1 memory.
Lv.4 Growlmon BT21-068 C Champion | Virus | Dark Dragon [Digivolve] Lv.3 w/[Guilmon] in name: Cost 2 [On Play] [When Digivolving] Delete 1 of your opponent's Digimon with 4000 DP or less. If this effect didn't delete, trash the top 2 cards of your deck. --- Inherited: [On Deletion] Gain 1 memory.