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 Digimon that aren't affected by effects won't be affected by it, therefore the gained effect won't trigger.
Lv.4 Greymon BT21-057 U Champion | Vaccine | Dinosaur/ADVENTURE [Digivolve] Lv.3 w/[Agumon] in name or w/[ADVENTURE] trait: Cost 2 [On Play] [When Digivolving] If you have [Tai Kamiya] or a Tamer with the [ADVENTURE] trait, give 1 of your opponent's Digimon "[Start of Your Main Phase] This Digimon attacks." until their turn ends. --- Inherited: <Reboot> (This Digimon also unsuspends in your opponent's unsuspend phase.)
Lv.4 Greymon BT21-057 U Champion | Vaccine | Dinosaur/ADVENTURE [Digivolve] Lv.3 w/[Agumon] in name or w/[ADVENTURE] trait: Cost 2 [On Play] [When Digivolving] If you have [Tai Kamiya] or a Tamer with the [ADVENTURE] trait, give 1 of your opponent's Digimon "[Start of Your Main Phase] This Digimon attacks." until their turn ends. --- Inherited: <Reboot> (This Digimon also unsuspends in your opponent's unsuspend phase.)
Lv.4 Greymon BT21-057 U Champion | Vaccine | Dinosaur/ADVENTURE {{DigivolveFromArchetypeOrTrait|2|3|Agumon|ADVENTURE}} [On Play] [When Digivolving] If you have [Tai Kamiya] or a Tamer with the [ADVENTURE] trait, give 1 of your opponent's Digimon "[Start of Your Main Phase] This Digimon attacks." until their turn ends. --- Inherited: <Reboot>.
Lv.4 グレイモン BT21-057 U 成熟期 | ワクチン種 | 恐竜型/アドベンチャー 【登場時】【進化時】自分の「八神太一」か特徴に「アドベンチャー」を持つ自分のテイマーがいるなら、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。 --- Inherited: ≪再起動≫(このデジモンは相手のアクティブフェイズでもアクティブになる)