No, you can't. A "by" condition can't be met if only some of the required actions are performed. The conditions for this [On Play] [When Digivolving] [When Attacking] effect require you to trash the specified number of cards under your Tamer.

Yes, you can.

No, it isn't deleted yet. Once all of the processing for this card's [On Play] [When Digivolving] [When Attacking] effect is resolved, then all of the Digimon with 0 DP are deleted at the same time. In addition, if an Option card is used by this effect, once the used Option card's [Main] effect has resolved and the Option card is trashed, then all Digimon with DPs of 0 will be deleted. If digivolving by Arts Digivolve, after digivolving, all of the Digimon with 0 DP are deleted.