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Let's say Countdown would take 4 attack damage. I use Omega Supreme's bot mode to have him take 2 of it, so Countdown only takes 2. Is the 2 damage Omega takes considered attack damage for the purpose of Countdown, and therefore move 1 damage off each of them onto the attacker?

It is attack damage. Omega's bot mode is basically a weaker version of Heroism, and that's been ruled that damage taken via Heroism is considered attack damage. This means that after resolving Omega, if Countdown still takes 2 or more attack damage and isn't KO'd, you get to resolve Countdown for both himself and Omega.
Source: Aequitas (2023-07-05)    【Link to Ruling】
Tags: Omega Supreme, Autobot Defense Base, Autobot Countdown, Aerospace Commander