I'm not a pro here, since i made my first tower cap forest in my last fort. As you i redirect a brook and just let a huge surface being covered by water, then wait for the evaporation. Since the brook is freezing in the map the redirect channels get emptyed often. So it seam that water don't damage the sapling neither the tower cap, neither the plants, since in those channel both grow. I'm also sure they actually grow underwater since i made my first water reservoir for my wells at the begining of the fort, then years later emptyed it and the floor was full of mushrooms and plants. In fact it seam to me that they grow a bit better when there in more water, like from 2/7 to 4/7 pressure, they then seam to pop up magically or it is just an effect i'm not sure since a fully grown tower cap will block the scare he is on.
But in general the way they grow seam random to me, some will pop up in a matter of second, other seam to take years to grow. I needed around 4/6 years to have the tower cap forest look the same density as you find it in a cavern. And sometime when i fill the reservoirs it look like the small tower caps are floating in the water its pretty funny, but i think its just an effect due to the water sometime covering, then showing the caps at low pressure.
And it seam they don't grow if you let a rock on the surface so i dump systematically all the rocks before i flood a surface, it look a lot nicer too, but you need some dwarf power.
But as i said i'm pretty new to this so i might be wrong.