It refers to a card that contains the specified text or icon in its name, traits, effects, inherited effects, (Rule), digivolution requirements, DNA digivolution, DigiXros requirements, burst digivolve, App Fusion, Link, or Assembly requirements. For example, a card with [Knightmon] in its text would include cards with the name [DarkKnightmon] and cards with the text [Knightmon] in their effects.

No, you can't. A "by" condition can't be met if only some of the required actions are performed. In order for the conditions of this card's {Trash} [Main] effect to be met, you must delete 2 of your level 5 or higher Digimon with [Myotismon] in their texts.

Yes, you must delete them whenever possible.