Rulings
Q1: If a Digimon has this card in its digivolution cards, does that mean that it isn't treated as "1 of your Digimon without X Antibody in its digivolution cards"?
That's correct. This card has a "Name: Also treated as X Antibody" rule, therefore a Digimon with this card in its digivolution cards isn't treated as "1 of your Digimon without X Antibody in its digivolution cards."
Q2: Does this card's inherited effect only activate if both X Antibody and a Digimon card are in the digivolution cards?
No, it activates even if only this card is in the digivolution cards.
Q3: If a Digimon with this card in its digivolution cards would be deleted other than by one of my effects, then an effect such as {{Decoy}} prevents it from being deleted, does this card's inherited effect activate?
Yes, it activates.
Q4: Can I use this card and then place this card in digivolution cards without digivolving 1 of my Digimon without X Antibody in its digivolution cards?
No, you can't. If digivolution isn't performed using this card's effect, you can't place this card in digivolution cards.
Data Sources
X Antibody Proto Form EX5-070 SR X Antibody While you have a Digimon, you may ignore this card's color requirements. [Security] Add this card to the hand. [Main] 1 of your Digimon without [X Antibody] in its digivolution cards may digivolve into a Digimon card with the [X Antibody] trait in your hand with the digivolution cost reduced by 1. If it did, place this card as its bottom digivolution card. (Rule) Name: Also treated as [X Antibody].
X Antibody Proto Form EX5-070 SR X Antibody While you have a Digimon, you may ignore this card's color requirements. [Main] 1 of your Digimon without [X Antibody] in its digivolution cards may digivolve into a Digimon card with the [X Antibody] trait in your hand with the digivolution cost reduced by 1. If it did, place this card as its bottom digivolution card. (Rule) Name: Also treated as [X Antibody]. --- [All Turns] When this Digimon would leave the battle area other than by one of your effects, from this Digimon's digivolution cards, return 1 Digimon card to the hand and place 1 [X Antibody] on top of your security stack. [Security] Add this card to the hand.
X Antibody Proto Form EX5-070 SR X Antibody While you have a Digimon, you may ignore this card's color requirements. [Security] Add this card to the hand. [Main] 1 of your Digimon without [X Antibody] or [link = X Antibody (Card Name)] in its digivolution cards may digivolve into a Digimon card with the [X Antibody] trait in your hand with the digivolution cost reduced by 1. If it did, place this card as its bottom digivolution card. --- [All Turns] When this Digimon would leave the battle area other than by one of your effects, from this Digimon's digivolution cards, return 1 Digimon card to the hand and place 1 [X Antibody] or [link = X Antibody (Card Name)] on top of your security stack.
X抗体PF EX5-070 SR X抗体 自分のデジモンがいる間、このカードは色条件を無視できる。 【セキュリティ】このカードを手札に加える。 【メイン】進化元に「X抗体」がない自分のデジモン1体を手札の特徴に「X抗体」を持つデジモンカードに支払う進化コスト-1で進化できる。そうしたとき、このカードをそのデジモンの進化元の下に置く。 〈ルール〉名称:「X抗体」としても扱う。 --- 【お互いのターン】このデジモンが自分の効果以外でバトルエリアを離れるとき、このデジモンの進化元から、デジモンカード1枚を手札に戻し、「X抗体」1枚を自分のセキュリティの上に置く。