You've mentioned the obvious discipline problems in all your critiques so far, and I think you're spot on. See, my story is set during the Fall (that is, the fall of my pseudo-roman empire). One of the factors that often tears down armies in this sort of situation is poorly-selected officers – people who were promoted for reasons of class, nepotism, or other sorts of corruption. So discipline has slowly fallen to the wayside. Also, Gaitu was terrible at training, but I haven't mentioned either of these things. I probably do need to change some of the situations so they're a little more plausible, and I also need to explain the discipline problems better. It's just, with everything else going on in the story I'm worried about an infodump (which i've already done a couple of times, as you pointed out). So on a re-write i'll explain it better – though I had actually planned to toss in an explanation next chapter, so maybe that'll do.
I guess my point is, you've given great feedback – you've pointed out several things that aren't quite believable, and I need to fix them – but the discipline thing is at least partially intentional. Some of the actions will make sense if I give more context, and some still won't so I'll have to ditch them.
Anyways, Zaisha commands the Immunes, who aren't regular soldiers, so she doesn't have any direct jurisdiction over the regulars. Also, like rome, my empire's army is actually based upon land-owners, with auxilians who hope to earn citizenship. because of the slave economy they didn't have the same sort of peasant structure as other nations might.