Rulings
Q1: My opponent has no Digimon with as much or more DP as this card in the battle area. I used this card to attack an opponent's Digimon, and the opponent's Digimon is deleted in the battle. Then, if an On Deletion effect on the deleted opponent's Digimon activates and my opponent now has a Digimon with as much or more DP as this Digimon, can I use the gained Piercing to perform a security check?
Yes, you can use Piercing to perform a security check. As soon as this card deletes an opponent's Digimon in battle, the Piercing gained from this card's Your Turn effect and the deleted Digimon's On Deletion effect will trigger simultaneously, then the turn player's Piercing will activate first. When Piercing activates, "perform check before End of Attack timing" processing will be pending, and the check will be performed immediately before the End of Attack timing, even if Piercing is lost after activation.
Q2: My opponent has no Digimon with as much or more DP as this card in the battle area. This card performs a security check, the Security effect on the 1st card checked activates, and my opponent now has a Digimon with as much or more DP as this card. Can I then use the Security Attack +1 gained from this card's Your Turn effect to perform a 2nd security check?
No, a 2nd security check can't be performed using that Security Attack +1. The Security Attack +1 gained from this card's Your Turn effect is lost as soon as your opponent has a Digimon with as much or more DP as this card, therefore you can't use the gained Security Attack +1 to perform a security check.
Data Sources
Lv.6 Callismon EX8-045 SR Mega | Virus | Dark Animal/NSo [DNA Digivolve] Green/purple Lv.5 + red/yellow Lv.5: Cost 0 [When Digivolving] Suspend 1 of your opponent's Digimon or Tamers. Then, return 1 of their suspended Tamers to the bottom of the deck. [Your Turn] For each color in this Digimon's digivolution cards, it gets +1000 DP. While your opponent has no Digimon with as much or more DP as this Digimon, this Digimon gains <Piercing> and <Security A. +1>.
Lv.6 Callismon EX8-045 SR Mega | Virus | Dark Animal/NSo [DNA Digivolve] Green/purple Lv.5 + red/yellow Lv.5: Cost 0 [When Digivolving] Suspend 1 of your opponent's Digimon or Tamers. Then, return 1 of their suspended Tamers to the bottom of the deck. [Your Turn] For each color in this Digimon's digivolution cards, it gets +1000 DP. While your opponent has no Digimon with as much or more DP as this Digimon, this Digimon gains <Piercing> and <Security A. +1>.
Lv.6 Callismon EX8-045 SR Mega | Virus | Dark Animal/NSo [Digivolve] Lv.5 w/[NSo] trait: Cost 3/undefined [When Digivolving] Suspend 1 of your opponent's Digimon or Tamers. Then, return 1 of their suspended Tamers to the bottom of the deck. [Your Turn] For each color in this Digimon's digivolution cards, it gets +1000 DP. While your opponent has no Digimon with equal or higher DP, this Digimon gets <Piercing> and <Security A. +1>.
Lv.6 カリスモン EX8-045 SR 究極体 | ウィルス種 | 魔獣型/NSo 〔ジョグレス〕緑/紫Lv.5+赤/黄Lv.5:コスト0 【進化時】相手のデジモン/テイマー1体をレストさせる。その後、レスト状態の相手のテイマー1体をデッキの下に戻す。 【自分のターン】このデジモンの進化元の色1色ごとに、このデジモンをDP+1000。このデジモンのDP以上の相手のデジモンがいない間、このデジモンは≪貫通≫と≪Sアタック+1≫を得る。