Q1: If a target Digimon of this card's When Digivolving effect later gains a digivolution card, can it now attack?
No, it can't. After this card's When Digivolving effect activates for a Digimon with no digivolution cards, that Digimon can't attack until the effect ends, even if it later gains a digivolution card.
Lv.6 Vikemon P-052 P Mega | Free | Beastkin [When Digivolving] If you have a Tamer in play, up to 3 of your opponent's Digimon with no digivolution cards can't attack until the end of your opponent's next turn. [When Attacking] [Once Per Turn] Return 1 of your opponent's Digimon with no digivolution cards to its owner's hand.
Lv.6 Vikemon P-052 P Mega | Free | Beastkin [When Digivolving] If you have a Tamer in play, up to 3 of your opponent's Digimon with no digivolution cards can't attack until the end of your opponent's next turn. [When Attacking] [Once Per Turn] Return 1 of your opponent's Digimon with no digivolution cards to its owner's hand.
Lv.6 Vikemon P-052 P mega | Free | Beastkin [When Digivolving] If you have a Tamer in play, up to 3 of your opponent's Digimon with no digivolution cards can't attack until the end of your opponent's next turn. [When Attacking] [Once Per Turn] Return 1 of your opponent's Digimon with no digivolution cards to its owner's hand.
Lv.6 ヴァイクモン P-052 P 究極体 | フリー | 獣人型 【進化時】自分のテイマーがいるとき、次の相手のターン終了時まで、進化元を持たない相手のデジモン3体まではアタックできない。【アタック時】[ターンに1回]進化元を持たない相手のデジモン1体を手札に戻す。