1.) You could put more millstones next to the one to use the excess power. Or extend your water wheel power plant and use it to power both your millstone
and your automatic magma pump stack.
2.) I prefer the querns at the moment, because I set up power plants really late. I.e. when the magma pump stack is already almost done. There might be a difference as of how much "milling" skillpoints the dwarves get for using a quern vs. a millstone. But the outcome is identical, anyway.
3.) I've never tried that, because the diagonal outflow past the floodgate would mean that the water/magma flows in very slowly past this pressure-breaker. Just try it yourself if your mechanic can approach the floodgate or not.
4.) Fortifications belong into 3 places.
- Part of your "critters stay outside!" design of your magma and cave water canals. Combine it with wall grates.
- The upper levels of your outer walls. You know, the walls on the inside of your moat. After a couple of years, you will probably want to have walls that are 2 z-levels high or more. Since you combine these higher levels with a 1 tile wide walkway, this is the ideal spot for archers of yours. Nobody from outside can shoot your cows and farmers, but the archers can shoot through them.
- Your shooting range for your marksman training. Naked goblins on one side, your archers on the other.
5.) Yes, with a moat, and a wall. Vultures and buzzard will still be able to get in, as well as flying titans. But it is more or less safe. Unless goblins can arrive on flying mounts during sieges? That would suck. :p
6.) You only need soil for the underground farms. Irrigation is not necessary anymore, that has been finally fixed in 31.22 or so.
A well is a nice thing to have. Place it near your hospital so that the dwarves who feed your injured dwarves don't have to walk all the way to the river. Also, dwarves will have baths near the well and be excited about it.
7.) Yes, but sometimes you need to keep a close eye on what the dwarves are doing. In your case, I would have tunneled the path to the river, installed floodgate and constructed floors (to prevent towercaps from growing in your canalization), installed a Wall Grate, a constructed wall and have dug a fortification into it. Then I would have sent in the miner on the ground level towards the river and just have channeled out that last tile between your floodgate/other means of keeping critters out. He would have done so without falling into the river, and your flooding project would have started to commence.
You also need to be wary of your dwarves walling themselves in during wall constructions. They prefer to stand to the north or to the west while constructing, and so it's easy for them to trap themselves. Just be aware of that and have an eye on them when you are building.
6.)