Q1: If this card's Your Turn effect triggers, can I choose to not activate it?
No, you can't. If the effect triggers, it must activate.
Q2: Can I choose different Digimon for the Digimon that gains Alliance and the Digimon that attacks for this card's Your Turn effect?
Yes, you can.
Q3: After my Digimon gains Alliance with this card's Your Turn effect, can I choose to not attack for the part of the effect after "then"?
Yes, you can.
Lv.5 MagnaAngemon ST21-06 SR Ultimate | Vaccine | Archangel/ADVENTURE [Digivolve] Lv.4 w/[ADVENTURE] trait: Cost 3 [On Play] [When Digivolving] Place 1 of your opponent's 6000 DP or lower Digimon as the top security card. For every 2 colors your Tamers have, add 2000 to this effect's DP maximum. [Your Turn] [Once Per Turn] When your other Digimon are played or digivolve, if any of them have the [ADVENTURE] trait, 1 of your Digimon gains <Alliance> for the turn. Then, 1 of your Digimon may attack. --- Inherited: <Alliance> (When this Digimon attacks, by suspending 1 of your other Digimon, this Digimon gains <Security A. +1> and the suspended Digimon's DP for the attack.)
Lv.5 MagnaAngemon ST21-06 SR Ultimate | Vaccine | Archangel/ADVENTURE [Digivolve] Lv.4 w/[ADVENTURE] trait: Cost 3 [On Play] [When Digivolving] Place 1 of your opponent's 6000 DP or lower Digimon as the top security card. For every 2 colors your Tamers have, add 2000 to this effect's DP maximum. [Your Turn] [Once Per Turn] When your other Digimon are played or digivolve, if any of them have the [ADVENTURE] trait, 1 of your Digimon gains <Alliance> for the turn. Then, 1 of your Digimon may attack. --- Inherited: <Alliance> (When this Digimon attacks, by suspending 1 of your other Digimon, this Digimon gains <Security A. +1> and the suspended Digimon's DP for the attack.)
Lv.5 MagnaAngemon ST21-06 SR Ultimate | Vaccine | Archangel/ADVENTURE [Digivolve] Lv.4 w/[ADVENTURE] trait: Cost 3 [On Play] [When Digivolving] Place 1 of your opponent's 6000 DP or lower Digimon as the top security card. For every 2 colors your Tamers have, add 2000 to this effect's DP maximum. [Your Turn] {{OncePerTurn}} When your other Digimon are played or digivolve, if any of them have the [ADVENTURE] trait, 1 of your Digimon gains <Alliance> for the turn. Then, 1 of your Digimon may attack. --- Inherited: <Alliance>.