

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?
Yes, you can. An effect that activates another effect lets you decide whether or not to process that other effect's "by" conditions.
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。 ---