Q1: My opponent has a Digimon with 13000 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 13000 DP or less, you must choose that card and delete it.
Q2: My opponent has a Digimon with 13000 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 13000 DP or less and an effect that prevents deletion?
Yes, you can.
Lv.6 Gallantmon P-186 P Mega | Virus | Holy Warrior/Royal Knight [Digivolve] Lv.5 w/[WarGrowlmon] in name: Cost 3 When this card would be played, if there is a Digimon with 13000 DP or more, reduce the play cost by 2 for every 5 total cards in both players' trashes. <Rush> <Blocker> [On Play] [When Digivolving] Delete 1 Digimon with 13000 DP or more. If this effect didn't delete, <Recovery +1 ≪Deck≫>.
Lv.6 Gallantmon P-186 P Mega | Virus | Holy Warrior/Royal Knight [Digivolve] Lv.5 w/[WarGrowlmon] in name: Cost 3 When this card would be played, if there is a Digimon with 13000 DP or more, reduce the play cost by 2 for every 5 total cards in both players' trashes. <Rush> <Blocker> [On Play] [When Digivolving] Delete 1 Digimon with 13000 DP or more. If this effect didn't delete, <Recovery +1 ≪Deck≫>.
Lv.6 Gallantmon P-186 P Mega | Virus | Holy Warrior/Royal Knight [Digivolve] Lv.5 w/[WarGrowlmon] in name: Cost 3 When this card would be played, if there is a Digimon with 13000 DP or more, reduce the play cost by 2 for every 5 total cards in both players' trashes. <Rush> <Blocker> [On Play] [When Digivolving] Delete 1 Digimon with 13000 DP or more. If this effect didn't delete, <Recovery +1 (Deck)>.