Yes, it activates. Until the end of your opponent's turn, 1 of their Digimon gains that effect, so the [End of Attack] effect will activate even if the affected Digimon digivolves or de-digivolves.

No, you can't end that attack. If the "by doing X" condition isn't met, the rest of the effect isn't processed. In this case, if you can't delete 1 of your other Digimon, then you can't end the attack.

After this effect activates, the current timing makes a transition to the end of attack timing. For example, if this effect activates during the attack declaration timing, it will make a transition to the end of attack timing. A transition to the counter timing or block timing won't occur, and the attack won't succeed.

Yes, such attacks can be ended. "End the attack" effects are effects that change the timing, they don't affect an attacking Digimon.

No, they can't. If an effect that ends the attack activates, the end of attack timing comes immediately. A transition to the counter timing won't occur, therefore a [Counter] effect can't be activated.

Yes, it activates.