1: Yes. As long as it's connected, it'll be running, allowing you to make watermills and such, and yes, there will by fish.
2: Sure, just make sure you don't offer that barrel/box instead or they'll get all pissy.
3: To be honest, I never worry about clothing. I really should, just to handle some of the anger, but it doesn't seem like much of a major source of unhappiness to me. From what I understand, once they get damaged clothes they'll just go and grab them on their own. You could buy them from traders, but setting up a basic pig tail fiber clothing industry is pretty simple and better in the long run.
4: If you have wheelbarrows set on the storage, then the dwarves will only haul with those barrows. If all the barrows are already in use, nobody else will haul. This is why I tend to make stockpiles small, but make plenty of them. In addition, you might also want to make sure the second stockpile is set to 'take' from the former aswell.
5: Depends on what you're looking for in gameplay. How many years in are you? If you just want a peaceful megaproject zone, you're doing just fine. If you're looking for serious combat and blood strewn halls, try building a castle on the surface the only houses your military, set them to train 24/7, and have the measly peasants oversee the all the necessities for life as the dwarven 1%. Or you could kill the elf traders in the name of dorfiness and see if they go to war with you.
Either way, usually it's about 4-5 years before anything major happens in my forts. About then is when I expect a large invasion or an unexpected megebeast, sometimes I just forget to feed people and they all starve to death, other times the most popular guy in the fort loses and arm and in response everyone else loses their minds for panic and despair.
It's entirely possible to go 50+ years with nothing exciting happening. At that point, I usually restart next to a tower or re-gen the world with more gobbos.