

Q1: Can I change the attack target of a Digimon that isn't affected by effects with an effect that "changes the attack target"?
Yes, you can.
Q2: Can this card's Opponent's Turn effect return itself to the bottom of the deck after activating it?
Yes, it can. If an effect activates, it is to be fully resolved even if the card that activated the effect is removed from that area during the processing.
Shota Kuroi BT26-092 U TS [Start of Your Main Phase] By trashing 1 [TS] trait card from your hand, <Draw 1> and gain 1 memory. [Opponent's Turn] When one of your opponent's Digimon attacks, by returning 1 of your [TS] trait Tamers to the bottom of the deck, you may change the attack target to 1 of your Digimon with the [TS] trait. --- [Security] Play this card without paying the cost.
Shota Kuroi BT26-092 U TS [Start of Your Main Phase] By trashing 1 [TS] trait card from your hand, <Draw 1> and gain 1 memory. [Opponent's Turn] When one of your opponent's Digimon attacks, by returning 1 of your [TS] trait Tamers to the bottom of the deck, you may change the attack target to 1 of your Digimon with the [TS] trait. --- [Security] Play this card without paying the cost.
Shota Kuroi BT26-092 U TS [Start of Your Main Phase] By trashing 1 [TS] trait card from your hand, <Draw 1> and gain 1 memory. [Opponent's Turn] When one of your opponent's Digimon attacks, by returning 1 of your [TS] trait Tamers to the bottom of the deck, you may change the attack target to 1 of your Digimon with the [TS] trait. --- [Security] Play this card without paying the cost.
黒井翔太 BT26-092 U TS 【自分のメインフェイズ開始時】自分の手札から、特徴「TS」を持つカード1枚を破棄することで、≪1ドロー≫、メモリー+1。 【相手のターン】相手のデジモンがアタックしたとき、特徴「TS」を持つ自分のテイマー1体をデッキの下に戻すことで、アタックの対象を特徴「TS」を持つ自分のデジモン1体に変更できる。 --- 【セキュリティ】このカードをコストを支払わず登場させる。