Q1: If I didn't trash digivolution cards of my opponent's Digimon with this card's Main effect, can I process the part of the effect after "then"?
The part of the effect after "then" will be processed regardless of whether or not you trash digivolution cards.
Summon Frost EX7-067 C [Main] Trash the top 2 digivolution cards of all of your opponent's Digimon. If this effect didn't trash, you may play 1 level 4 or lower Digimon card with the [Ice-Snow] trait from your hand without paying the cost. Then, until the end of your opponent's turn, none of their Digimon with no digivolution cards can attack. --- [Security] Activate this card's [Main] effects.
Summon Frost EX7-067 C [Main] Trash the top 2 digivolution cards of all of your opponent's Digimon. If this effect didn't trash, you may play 1 level 4 or lower Digimon card with the [Ice-Snow] trait from your hand without paying the cost. Then, until the end of your opponent's turn, none of their Digimon with no digivolution cards can attack. --- [Security] Activate this card's [Main] effects.
Summon Frost EX7-067 C [Main] trash the top 2 digivolution cards of all of your opponent's Digimon. If this effect didn't trash, you may play 1 level 4 or lower Digimon card with the [Ice-Snow] trait from your hand without paying the cost. Then, none of their Digimon with no digivolution cards can attack until the end of their turn. --- [Security] Activate this card's [Main] effect.
サモンフロスト EX7-067 C 【メイン】相手のデジモン全ての進化元を上から2枚破棄する。この効果で破棄しなかったなら、自分の手札から、特徴に「氷雪型」を持つLv.4以下のデジモンカード1枚をコストを支払わずに登場できる。その後、相手のターン終了まで、進化元を持たない相手のデジモン全てはアタックできない。 --- 【セキュリティ】このカードの【メイン】効果を発揮する。