

Q1: Can I use this card's Main effect to place just 1 of the specified cards in digivolution cards?
No, you can't. A "by" condition can't be met if only some of the required actions are performed. The conditions for this Main effect require you to place 1 of each of the specified cards as digivolution cards.
Queen of Thorns BT26-098 U DATA SQUAD When this card would be used, by trashing the bottom face-down card from under any of your Tamers, reduce the cost by 2. [Main] By placing 1 [Sunflowmon] and 1 [Lilamon] from your trash as 1 of your [Lalamon]'s bottom digivolution cards, that Digimon may digivolve into [Rosemon] in the hand, ignoring digivolution requirements and without paying the cost. --- [Security] You may play 1 [Lalamon] or [Yoshino Fujieda] from your hand or trash without paying the cost. Then, add this card to the hand.
Queen of Thorns BT26-098 U DATA SQUAD When this card would be used, by trashing the bottom face-down card from under any of your Tamers, reduce the cost by 2. [Main] By placing 1 [Sunflowmon] and 1 [Lilamon] from your trash as 1 of your [Lalamon]'s bottom digivolution cards, that Digimon may digivolve into [Rosemon] in the hand, ignoring digivolution requirements and without paying the cost. --- [Security] You may play 1 [Lalamon] or [Yoshino Fujieda] from your hand or trash without paying the cost. Then, add this card to the hand.
Queen of Thorns BT26-098 U DATA SQUAD When this card would be used, by trashing the bottom face-down card from under any of your Tamers, reduce the cost by 2. [Main] By placing 1 [Sunflowmon] and 1 [Lilamon] from your trash as 1 of your [Lalamon]'s bottom digivolution cards, that Digimon may digivolve into [Rosemon] in the hand, ignoring digivolution requirements and without paying the cost. --- [Security] You may play 1 [Lalamon] or [Yoshino Fujieda] from your hand or trash without paying the cost. Then, add this card to the hand.
茨の女王 BT26-098 U セイバーズ このカードを使用するとき、自分のテイマーの下にある裏向きのカードを下から1枚破棄することで、支払うコスト-2。 【メイン】自分のトラッシュから、「サンフラウモン」と「ライラモン」1枚ずつを自分の「ララモン」1体の進化元の下に置くことで、そのデジモンを手札の「ロゼモン」に進化条件を無視してコストを支払わず進化できる。 --- 【セキュリティ】自分の手札/トラッシュから、「ララモン」/「藤枝淑乃」1枚をコストを支払わず登場できる。その後、このカードを手札に加える。