You're running a speed nuke team. What's the speed of your team?
Speed nuke teams require two things:
1) Obviously, you need to win the speed race. So the speed of your Khatun is critically important. I would guess you'll need at least 250 speed at your rating.
2) Less obvious is that the rest of your team needs to be fast enough that you can prevent cut-in. What this means is, after your Khatun does her speed boost move, the rest of your team needs to be able to act immediately afterwards.
Valkyrie helps, since she can mess with their turn meter manipulating, and can sometimes let you win fights you'd otherwise lose.
Regarding Zargala vs Kael - Zargala's multiplier on her A3 is significantly lower than that of Kael. Even if all three of her hits crit, she'll still be doing much less damage than Kael will be. She makes up for this with her amazing A2, but that's dependent on you actually getting the kill with it, which may not always be possible. Couple this with the fact that Kael is easier to gear and book, and I'd say you probably should stick with Kael - although if he's geared for CB that may not be the case. You'll have to make that judgement call yourself.
One thing which may help you though is to replace Valkyrie with Apothecary. The idea here is that you get a double speed boost, which lets you make up for having a slower nuker. Then, you can avoid using speed boots on the nuker, and really gear them purely for damage. This also helps if your Khatun is particularly fast. This is sort of the new meta now - almost all teams at higher ranks are running either two speed boosters, or comps that specifically counter those two speed booster teams.