It looks like you forgot that Simon can Chaos Bolt, and apparently misread Edgin's damage as 9.35 somehow rather than d6+2d10+3 (17.5). I'm not trying to be snide or "win an argument", but in a discussion about numbers, having the right numbers matters so I'll redo the numbers.
Chaos bolt is at disadvantage in melee (it's a ranged attack), so in melee, Simon is worse.
Edgin has that add on, but he's hitting a lot less than Holga that's where it gets lower.
But sure, it's not lower enough considering what it's modeling.
Those add on damages for Simon and Edgin are no necessary.
I am not counting Reckless because without Rage it's even more suicidal than normal. (See below for Reckless attack and defense numbers.)
She get's hit A LOT in the movie - they make a point of showing it. So she's clearly going reckless all the time.
The problem is, for some reason, they didn't include the barbarians signature rage ability?!?
With that, reckless is worth it. Without, it's suicidal. That to me, makes little sense!
Therefore WotC Holga (3x +7 for d12+4) vs. AC 16 hits on 9 or higher, doing 0.65 * 3d12 + 0.6 * 12 (19.875) damage per round.
WotC Edgin (2x +6 for d6+3+2d10) vs. AC 16 hits on a 10 or higher, doing 0.6 * (4d10+2d6) + 0.55 * 6 (20.7) damage per round.
WotC Simon (3x +6 for 2d10+3) vs. AC 16 hits on a 10 or higher, doing 0.6 * 6d10 + 0.55 * 9 (24.75) damage per round.
Holga has the lowest damage of the three!
Chaos bolt is ranged. If he uses it in melee, it's at disadvantage - that drops Simon's numbers significantly.
If Holga did attack recklessly, she'd bring her damage up to 0.9375 * 3d12 + 0.84 * 12 (28.36), barely outdamaging Simon while simultaneously making herself the squishy of the party. She'd take 28.9/120 (24% of her HP, lasting about 4 rounds) every round against 6 CR 1/8 Guards, whereas e.g. Simon would only take 14.55/99 (15% of his HP, lasting about 7 rounds).
In the scene with Holga tanking the guards ("Are you sure I can't help?" "I'm sure." "You don't have to sound so certain.") it's absurd to have Holga go down before Simon would have if he'd stayed. That's why I don't count Reckless--and even with Reckless, the bigger point is that Holga isn't supposed to barely outdamage Edgin and Simon. She's supposed to blow them out of the water to the extent that she makes them look like noncombatants. At least, if you want characters who match the movie.
Again, the true problem is that Holga does not include a rage ability - THAT makes no sense. It would totally fit the movie AND it would play by the rules.
Well, that and the fact that she's the only one (Doric doesn't get it either but we're not talking about her here) that doesn't get a large amount of extra damage, basically out of nowhere. Which doesn't fit the movie even. Again, I think it's an odd attempt at unnecessary balance.