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>.