Thanks for playing!
Deploying troops mid-battle was a bit of an afterthought, I don't think it really has much use. Units get added to the back of the queue, so after they're placed, every other unit will go before them, this might be what you're seeing? Unless there is a bug I haven't reproduced myself. And yes, the final battle is pretty tough, and dependent on how well the mangonel (mango OP) does which can be a bit RNG.
Thanks again for the feedback :)