

Q1: Do I have to choose to activate this card's When Attacking effect if it triggers?
Yes, you must choose to activate it. Even if you don't have a face-up Zaxon trait Digimon card in your security stack, it counts toward the Once Per Turn limit.
Q2: I only have a face-up BT23-045 TigerVespamon ACE in my security stack. If I use this card's When Attacking effect to activate that BT23-045 TigerVespamon ACE's On Play effect, can I choose to not process the On Play effect's "by" condition?
No, you can't. If the effect will activate, you must process the "by" condition. If you have a Royal Base or Zaxon trait card in the trash, you must place 1 of those cards face up as the bottom security card. If you don't have a Royal Base or Zaxon trait card in the trash, you can choose to place one from the hand face up as the bottom security card.
Lv.6 Machinedramon BT23-060 U Mega | Virus | Machine/Zaxon/CS <Security A. +1> <Reboot> [On Play] [When Digivolving] <De-Digivolve 1> 1 of your opponent's Digimon. Then, delete 1 of their Digimon with 8000 DP or less. [When Attacking] [Once Per Turn] Activate 1 [On Play] effect on a face-up [Zaxon] trait Digimon card in your security stack as an effect of this Digimon. ---
Lv.6 Machinedramon BT23-060 U Mega | Virus | Machine/Zaxon/CS [Digivolve] Lv.5 w/[CS] trait: Cost 3 <Security A. +1> <Reboot> [On Play] [When Digivolving] <De-Digivolve 1> 1 of your opponent's Digimon. Then, delete 1 of their Digimon with 8000 DP or less. [When Attacking] [Once Per Turn] Activate 1 [On Play] effect on a face-up [Zaxon] trait Digimon card in your security stack as an effect of this Digimon. --- -
Lv.6 Machinedramon BT23-060 U mega | Virus | Machine/Zaxon/CS [Digivolve] Lv.5 w/[CS] trait: Cost 4 <Security A. +1> <Reboot> [On Play] [When Digivolving] <De-Digivolve 1> 1 of your opponent's Digimon. Then, delete 1 of their Digimon with 8000 DP or less. [When Attacking] [Once Per Turn] Activate 1 [On Play] effect on a face-up [Zaxon] trait Digimon card in your security stack as an effect of this Digimon.
Lv.6 ムゲンドラモン BT23-060 U 究極体 | ウィルス種 | マシーン型/ザクソン/CS ≪Sアタック+1≫≪再起動≫ 【登場時】【進化時】相手のデジモン1体を≪退化1≫。その後、DP8000以下の相手のデジモン1体を消滅させる。 【アタック時】[ターンに1回]自分のセキュリティの特徴に「ザクソン」を持つ表向きのデジモンカード1枚の【登場時】効果1つをこのデジモンの効果として発揮する。 ---