Q1: Can I use this card’s On Play When Digivolving effect to place 1 level 5 card with Dark Masters in its traits as a digivolution card even when my opponent doesn't have a level 5 or lower Digimon?
Yes, you can.
Q2: Can I use this card's When Attacking effect on a Digi-Egg card in my opponent's trash?
Yes, you can. However, if a Digi-Egg card is the target of an effect that places it at the bottom of the deck, it will be placed at the bottom of the Digi-Egg deck.
Lv.6 Gulfmon BT17-070 R Mega | Virus | Dark Animal [Digivolve] Lv.5 w/[Dark Masters] in text: Cost 3 [On Play] [When Digivolving] By placing 1 level 5 card with [Dark Masters] in its text from your hand or trash as this Digimon's bottom digivolution card, delete 1 of your opponent's level 5 or lower Digimon. [When Attacking] By returning 7 cards from your opponent's trash to the bottom of the deck, unsuspend this Digimon.
Lv.6 Gulfmon BT17-070 R Mega | Virus | Dark Animal [Digivolve] Lv.5 w/[Dark Masters] in text: Cost 3 [On Play] [When Digivolving] By placing 1 level 5 card with [Dark Masters] in its text from your hand or trash as this Digimon's bottom digivolution card, delete 1 of your opponent's level 5 or lower Digimon. [When Attacking] By returning 7 cards from your opponent's trash to the bottom of the deck, unsuspend this Digimon.
Lv.6 Gulfmon BT17-070 R Mega | Virus | Dark Animal [Digivolve] Lv.5 w/[Dark Masters] in text: Cost 3 [On Play] [When Digivolving] By placing 1 level 5 card with [Dark Masters] in its text from your hand or trash as this Digimon's bottom digivolution card, delete 1 of your opponent's level 5 or lower Digimon. [When Attacking] By returning 7 cards from your opponent's trash to the bottom of the deck, unsuspend this Digimon.
Lv.6 ガルフモン BT17-070 R 究極体 | ウィルス種 | 魔獣型 「ダークマスターズ」の記述があるLv.5:コスト3 【登場時】【進化時】自分の手札/トラッシュから、「ダークマスターズ」の記述があるLv.5のカード1枚をこのデジモンの進化元の下に置くことで、Lv.5以下の相手のデジモン1体を消滅させる。 【アタック時】相手のトラッシュのカード7枚をデッキの下に戻すことで、このデジモンをアクティブにする。