No, you can't. A "by" condition can't be met if only some of the required actions are performed. The conditions for this inherited effect require you to trash the specified number of digivolution cards.

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.