Rulings
Q1: Does this card’s When Digivolving effect force both players to trash the top 2 cards of their decks?
Yes. Both players must trash 2 cards each, if possible.
Q2: Can I choose to not return the cards to my hand for this card's When Digivolving effect when I have 1 Guilmon or 1 card with Growlmon or Gallantmon in its name in my trash?
Yes, the effect reads ""you may return ... to your hand,"" therefore you may choose to not return cards to your hand. However, even in such cases, both players must trash 2 cards from the tops of their decks.
Q3: Can I choose to not return the cards to my hand for this card's inherited effect when I have 1 Guilmon or 1 card with Growlmon or Gallantmon in its name in my trash?
Yes, the effect reads "you may return ... to your hand," therefore you may choose to not return cards to your hand.
Data Sources
Lv.4 BlackGrowlmon EX4-008 U Champion | Virus | Dark Dragon [When Digivolving] Trash the top 2 cards of both players' decks. Then, you may return 1 [Guilmon] or 1 card with [Growlmon] or [Gallantmon] in its name from your trash to your hand. --- Inherited: [On Deletion] You may return 1 [Guilmon] or 1 card with [Growlmon] or [Gallantmon] in its name from your trash to your hand.
Lv.4 BlackGrowlmon EX4-008 U Champion | Virus | Dark Dragon Digivolve: 2 from Lv.3 w/[Guilmon] in name [When Digivolving] Trash the top 2 cards of both players' decks. Then, you may return 1 [Guilmon] or 1 card with [Growlmon] or [Gallantmon] in its name from your trash to your hand. --- Inherited: [On Deletion] You may return 1 [Guilmon] or 1 card with [Growlmon] or [Gallantmon] in its name from your trash to your hand.
Lv.4 BlackGrowlmon EX4-008 U
champion | Virus | Dark Dragon
{{DigivolveFromArchetypes|2|3|Guilmon}}
[When Digivolving] Both players trash the top 2 cards of their decks. Then, you may return 1 [Guilmon] or 1 card with [Growlmon] or [Gallantmon] in its name from your trash to your hand.
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Inherited: [On Deletion] You may return 1 [Guilmon] or 1 card with [Growlmon] or [Gallantmon] in its name from your trash to your hand.Lv.4 ブラックグラウモン EX4-008 U 成熟期 | ウィルス種 | 魔竜型 【進化時】お互いのデッキの上から2枚破棄する。その後、自分のトラッシュから、「ギルモン」か名称に「グラウモン」か「デュークモン」を含むカード1枚を手札に戻せる。 --- Inherited: 【消滅時】自分のトラッシュから、「ギルモン」か名称に「グラウモン」か「デュークモン」を含むカード1枚を手札に戻せる。