Rulings
Q1: I have a Digimon that gained Security Attack +1 from another card’s effect, and which also gained Piercing from this card’s inherited effect. I attack an opponent's Digimon with it, and perform a security check with Piercing. The first card checked has a De-Digivolve 1 Security effect, which trashes the top card of my attacking Digimon, turning it into a Digimon without Machine or Dragonkin. This causes the Digimon to also lose Piercing. Do I still perform the second check?
Yes. Piercing has already activated. Perform the additional check, assuming your Digimon still has Security Attack +1.
Q2: I attack my opponent with this Digimon. If they block it with one of their Digimon, does this card's effect activate?
Yes, it does. When Attacking effects activate before your opponent gets to activate Blocker.
Data Sources
Lv.5 MetalGreymon EX1-008 R Ultimate | Vaccine | Cyborg [When Attacking] When this Digimon attacks a player, delete 1 of your opponent's Digimon with 4000 DP or less. --- Inherited: [Your Turn] While this Digimon has [Machine] or [Dragonkin] in its traits, it gains <Piercing>. (When this Digimon attacks and deletes an opponent's Digimon and survives the battle, it performs any security checks it normally would.)
Lv.5 MetalGreymon EX1-008 R Ultimate | Vaccine | Cyborg [When Attacking] When this Digimon attacks a player, delete 1 of your opponent's Digimon with 4000 DP or less. --- Inherited: [Your Turn] While this Digimon has [Machine] or [Dragonkin] in its traits, it gains <Piercing>. (When this Digimon attacks and deletes an opponent's Digimon and survives the battle, it performs any security checks it normally would.)
Lv.5 MetalGreymon EX1-008 R ultimate | Vaccine | Cyborg [When Attacking] When this Digimon attacks a player, delete 1 of your opponent's Digimon with 4000 DP or less. --- Inherited: [Your Turn] While this Digimon has the [Machine] or [Dragonkin] trait, it gains <Piercing>.
Lv.5 メタルグレイモン EX1-008 R 完全体 | ワクチン種 | サイボーグ型 【アタック時】プレイヤーにアタックしたとき、DP4000以下の相手のデジモン1体を消滅させる。 --- Inherited: 【自分のターン】このデジモンが特徴に「マシーン型」か「竜人型」を持つ間、このデジモンは≪貫通≫（このデジモンがアタックしたバトルで相手のデジモンだけを消滅させたとき、このデジモンはセキュリティをチェックする）を得る。