I'm not talking about whether or not he's sufficiently powerful.
I'm talking about whether the game makes sense, whether the game makes it easy to suspend disbelief.
There are ways to do that without making any changes to game mechanics at all: If Fives was cybernetic ally enhanced, and this cybernetic "threat processor" that helped his situational awareness and counter-attacks ignored immobile objects, because why would a battlefield threat processor try to make you give more attention to immobile objects?
THAT would be a reason to double-tap characters affected by Speed Down, but not by Stun.
The problem is that the game doesn't give us any of that. Good color text can make something that seems ridiculous and nonsensical (Fives can't double tap a motionless lump, but he can double-tap a QGJ-led Yoda if Yoda happened to be spending 1.2 seconds affected by Speed Down) seem obvious and logical (why would a threat processor help you attack motionless things?).
Frankly, I'd be happier with more text in game, but since that doesn't seem to be coming, making 5s double-tap stunned characters would work as well.
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ALSO: It would make absolutely no difference in the scenario you mentioned, since Fives can't stun and any stunning teammates Fives had were defeated.