Second, we've seen that Church is MUCH faster than Alice (being able to cross several paces before Alice can move her arm two feet(, who in turn is - if not faster - at least stronger than Sedna. So either 1, why wasn't Church running at anywhere near top speed, or 2, how dod Sedna suddenly run faster than him?
Both Pate and Church were anticipating an attack when Alice moved to kill Pate and were ready for her. If Church's attention had been on Sedna, she probably would not have gotten to him in one piece - make that two pieces. But, he was focused on (presumably) Alice and probably thinking up a 5,000 year torment to inflict upon her and didn't remember/bother to check his six. (Note the contrast between Church's normal flat affect poker face versus his shark smile whenever he's attacking Alice.) For Sedna it was now-or-never - she saw her chance to kill the bastard and went for it.
Take a closer look at that comic. It's hard to tell exactly the real distances, but based off of the character's scale relative to one another, Alice is a couple of paces - five or six feet- at most behind Pate, and Church is WELL behind either of them, at least ten feet behind Alice.
In the time it takes Alice to leap forward five feet at top speed, Church doesn't just cross all the way over, and grab Alice's arm, he does it like it's
nothing. He blurs, and suddenly his hand is gripping hers with enough strength to stop all of her forward momentum, without having to exert himself in the slightest - He's not leaning into the grab, or pushing her, he's just standing there without any leverage and he's still able to stop her cold.
Given this, Church is
at least three times faster than Alice, and it is implied to be much more.
Meanwhile, when Church dives after Alice, Sedna has time to - While standing in one spot a couple of paces behind Church - Rip off her own arm, break it into two pieces with her mouth, and then chase down Church, when Church is sprinting forward at top speed in order to (presumably) finish off Alice.
I'm not saying that Sedna couldn't have snuck up behind him and/or taken him by surprise while he pounded Alice in the face over and over, I'm saying that, given the information we have learned from the comic, she couldn't possibly have chased him down. (Also: It was apparently a very loud crack when she broke her arm in half, which would presumably be something worth checking out, even to him.)
I would say that if we were to crudely equate the super soldiers to something similar, then Church is a dreadnought, Alice is a cruiser and Sedna is a destroyer. Church is the powerhouse that needs to build up speed, Alice is the jack-of-all-trades, while Sedna is brings firepower beyond her comparative size. Essentially, The destroyer rammed the dreadnought and took it down.
As for Laridia? Easy. She's a modern day aircraft carrier compared to their WWI prowess. As in she's got the reach and damage potential the others can only dream of.
That doesn't really solve much, though. My point about Laridia was easily the weakest, so I could buy that she just didn't react quickly enough before the melee went down, but we've clearly seen that these characters are effectively indestructible, with Church being completely unphased by literally everything else up to this point. Alice uses a weapon that we can presume is specifically designed for her to use against other super-soldiers, and then hits him with an orbital space cannon exterminatus beam laser, and all it does is trap him Rita Repulsa style. A splintery bone should not be better at cutting through armor than an actual weapon.
And even if Sedna is faster than Alice, there's no reason for her to be THAT MUCH faster, and we've clearly seen that Church does not need to build up speed - He catches Alice without breaking a sweat, literally blurring through the air. According to the information given by Sedna, she's a support grunt, while Alice and Church are much more powerful. Sedna is
not even a soldier, or at least not one that was designed to see front-line combat.