Yes, it can digivolve.

Yes. After this card's [Your Turn] effect activates, then you perform the security check for <Piercing>. This card's [Your Turn] effect and <Piercing> trigger simultaneously, but the activated <Piercing> is processed immediately before the attack ends, therefore <Security A. +1> is first gained from this card's [Your Turn] effect, then you perform the security check for <Piercing>.

Yes, it can attack. When digivolving into this card, the turn end conditions won't be met until all of the resulting processing has been resolved. In this case, this card's [When Digivolving] effect, [Your Turn] effect, and any effects triggered by them must first be resolved, and then the turn end conditions will be met. Then, the end of the turn timing will occur and BT18-088 [Takuya Kanbara & Koji Minamoto]'s [End of Your Turn] effect will trigger.

Yes. For each color in this Digimon's digivolution cards, you choose and trash 1 digivolution card from your opponent's Digimon and suspend 1 of their Digimon.