

Q1: Can I use this card's On Play When Digivolving effect to give "Start of Your Main Phase This Digimon attacks" to a Digimon that isn't affected by effects?
You can give that effect to such Digimon, but if the Digimon isn't affected by effects upon the trigger timing, the gained effect won't trigger.
Q2: Can I trash just 1 digivolution card for the conditions of this card's All Turns effect?
No, you can't. A "by" condition can't be met if only some of the required actions are performed. The conditions for this All Turns effect require you to trash the specified number of digivolution cards.
Q3: Does this card's All Turns effect trigger when either my Digimon or my opponent's Digimon attacks?
Yes, it triggers.
Lv.6 Blastmon EX10-034 R Mega | Vaccine | Mineral/Bagra Army <Collision> <Fragment《3》> <Blocker> [On Play] [When Digivolving] Until your opponent's turn ends, give 1 of their Digimon "[Start of Your Main Phase] This Digimon attacks." [All Turns] [Once Per Turn] When Digimon attack, by trashing any 2 of this Digimon's digivolution cards, this Digimon gains <Security A. +1> and +3000 DP until your turn ends. ---
Lv.6 Blastmon EX10-034 R Mega | Vaccine | Mineral/Bagra Army <Collision> <Fragment《3》> <Blocker> [On Play] [When Digivolving] Until your opponent's turn ends, give 1 of their Digimon "[Start of Your Main Phase] This Digimon attacks." [All Turns] [Once Per Turn] When Digimon attack, by trashing any 2 of this Digimon's digivolution cards, this Digimon gains <Security A. +1> and +3000 DP until your turn ends. DigiXros -2: 3 Digimon cards w/[Bagra Army] trait When this would be played, you may place specified cards from your hand/battle area under it. Each one reduces the play cost. --- -
Lv.6 Blastmon EX10-034 R mega | Vaccine | Mineral/Bagra Army <Collision> (During this Digimon's attack, all of your opponent's Digimon gain <Blocker>, and the opponent blocks if able.) <Fragment ≪3≫> <Blocker> [On Play] [When Digivolving] Until your opponent's turn ends, give 1 of their Digimon "[Start of Your Main Phase] This Digimon attacks." [All Turns] [Once Per Turn] When Digimon attack, by trashing any 2 of this Digimon's digivolution cards, this Digimon gains <Security A. +1> and +3000 DP until your turn ends. DigiXros -2: 3 Digimon cards w/[Bagra Army] trait
Lv.6 ブラストモン EX10-034 R 究極体 | ワクチン種 | 鉱物型/バグラ軍 ≪衝突≫≪フラグメント《3》≫≪ブロッカー≫ 【登場時】【進化時】相手のターン終了まで、相手のデジモン1体に「【自分のメインフェイズ開始時】このデジモンでアタックする。」の効果を与える。 【お互いのターン】[ターンに1回]デジモンがアタックしたとき、このデジモンの進化元を選んで2枚破棄することで、自分のターン終了まで、このデジモンは≪Sアタック+1≫を得て、DP+3000。 ---