Seize by Force is used whenever the battle is coming to blows with both sides looking to waste the other. If words are involved and the situation is tense, but bullets aren't flying, that's when go aggro comes into play. Your first and last two uses of seize by force were unnecessary.
When a seize by force is rolled, both sides suffer harm if harm has been established by their weaponry. If one of them has a sword and the other a gun, the gun's harm is probably dealt first unless the guy with the sword started the seize roll already next to the gunner. If they both have weapons in the same range, then I'd just say both sides drop the blows on the other-side and that's that.
You may want to consider the effects of the option impress, dismay, or frighten. For example: having the shotgun cleave off half the guy is certainly a SIGHT to be seen, yet really a shotgun might be hard pressed to do that. GO with whatever visceral imagery you and your players enjoy.. but there's always that moment when the bullets tear through Dremmer that the other two's faces go white and you show that camera on their Human response to their buddy dying right there on the street. Running might be the option, but so might screaming for mercy, or start blabing out words that might save their life. As well, in that moment when both of them watch their buddy hit the ground, being impressed, dismayed, or frightened, might keep them from reacting fast enough and running off.
White was on the ground, shocked. If Gritch wanted to drop down on top of him, and he was already sold on fleeing this fucking combat. I'd just say when he starts to repeatedily stab the guy with a knife, that's that. Players have the advantage, the main leader's down, the rest is just mop up work. Unless the threat's still active, and you're looking to trade more harm for harm, dont worry about making them roll. If they do it, they do it. If white being on the ground reached for his weapon and screamed, LEAVE ME ALONE! And Gritch still went to jump on top of him, then it's a seize by force moment, cause harm between the knife and White's weapon can be traded again as established.
Same goes for the Gunlugger shooting the guy in the leg. If she is using the same shotgun that blasted the other guy's entire torso off... I'd say at the bare minimum shooting him in the leg deals the FULL HARM. You don't deal less harm then established unless they've taken the Quarantine move called disciplined engagement. She could either switch to a light handgun and give him a quick pow pow, or just run out after him and beat him down with the shotgun's butt dealing s harm and drag his ass back. If she does it, she does it. Shooting someone in the leg tends to kill them a lot of the time however, you've a lot of big arteries down there. Shotguns aren't weapons that deal in less then lethal damage.
If you as an MC don't want to just decide that she catches the fleeing target, make her roll a act under fire roll. She's acting under the risk of him escaping, and he's already running, maybe even ducking and weaving trying to keep under cover? Since he isn't fighting back, and you are still asking for a roll, use the results something on the lines of:
Roll Act Under Fire, +cool (or hard if battle-hardened). On a 10+ You manage to nick him and he drops to the ground. He suffers 1 less harm then normal. On a 7–9, You manage to find a couple opportunities to make the shot, but he's keeping well under cover. You have to make a snap decision, take the shot and risk hitting something vital? Or let him off this time. So if she takes the shot, deal full harm, deal with the result. If she lets him run off, then that's that, he'll be afraid of her for a long while. On a miss, she takes the shots, but shit just keeps getting in the way. He makes an escape and you've lost sight of him. You might notice that he's running in the gang's direction, but he'll get to help before you'll get to him. You're involved now.
Of course, the Gunlugger could have always just aimed her gun at Preen as he starts to run off and said something like. "WOOO There. Where do you think you're going? I didn't give you permission to leave yet." Let the Gunlugger roll Go Aggro on the guy. That would give you lots of options that will give the Gunlugger what she wants on a Hit, a live target that isn't getting away. If she misses the roll, then the guy not only starts running, but he also gets away. Don't follow the failed Go Aggro with a Seize by Force. Make the miss change the narrative, make a move that moves the story forward. Keep in mind that Preen is impressed dismayed, or frightened, always make that response work to the characters benefit, since they choose it as part of their successful move. So if the Gunlugger hits on the Go Aggro, he knows she'll blow him into tiny weeny pieces too, and he doesn't want that. Why would he risk making her mad?
Daniel posted a bunch of this right before I finished it, so we say the same thing a few times. I figure you can read both and see the perspectives, and so I'll leave mine as written.