First off, stop making your dwarves drink water. It's undwarvenly and inhumane. Armok frowns on your shenanigans. Also, they take more breaks and longer breaks if they haven't had a good honest tankard of booze lately.
anyway, i have a question about rivers. is it possible to divert or reroute a river entirely? i think having an entire river under your control would be awesome.
Not entirely. You can control a river on your fortress map any way you want, and when you get a bit more experience with the game, I highly recommend you try to dam off an actual river (as opposed to a "brook"), it's a lot of fun and a huge engineering project.
However, whatever efforts you make on the fortress map won't affect the world map. Even if you divert a mighty river and flood a nearby canyon, this won't be reflected on the world map, it will look the same as always and any rivers you've messed with will magically continue on their original courses once they leave your fortress area.
all my token effort is repeatedly undermined by my own dwarves though, they seem to purposefully leave large gaps in the wall for no reason, and on the points where the elevation changes leaves eery gaps as units can just walk on top of the less elevated wall and over to the other side. my orders to patch up these oversights is ignored completely at the best of times, and suspended by my own dwarves at the worst.
The most common reason for dwarves to suspend tasks is because they saw a fearsome woodchuck or deer (or possibly one of those damn monkeys) and got scared. You can manually unsuspend the task by moving your cursor over the unbuilt wall and pressing "s", I believe. You may have to do this a few times before you get your walls built completely.
i need some help trading. the elves came to me wanting to trade, but i can't figure out how to do it. I understand moving things over their to trade, and entering the trade screen, but when it comes to the actual exchanging part, i'm stuck. i offer him things that are equal in value to what i want, but i can't do anything.
Once you have stuff marked to trade on both sides, you press "t" to trade, I believe. However, you have to offer more than you're getting until your trader becomes a lot more experienced. Don't worry if the traders won't give you a fair deal, this happens to everybody.
also, some rhesus macaque monkey showed up at the same time as those elves, and they started stealing things from the elven traders! they disappeared mysteriously after that, and when i was looking through the trade screen, i noticed "rhesus macaque cages" and i concluded that the elves had caught them and were now trying to sell them to me.
No, the elves caught those rhesus macaques in their homeland and brought them thousands of miles to YOU, because clearly you don't have ENOUGH rhesus macaques already. Traders will fight to defend themselves, but they don't capture anything.
while i haven't been officially invaded yet, i have been bothered by kobold thieves and goblin snatchers or something, and i'm pretty sure those are bad. i've assigned a new migrant (who brought his own crossbow) as a soldier (i think!), but when they show up, he isn't kicking their ass. what do i do to order him to kick ass?
Well, first you have to (shift-A) Activate him so that he's a "Recruit" instead of a "Peasant" or whatever. Then you have to go into the "Military" screen (by pressing "m", naturally) and over at the right side, there's an area that shows what weapon he's equipped with. Change that to "crossbow". Make sure there's an unequipped crossbow and some bolts somewhere for him to pick up. I'd also recommend giving him some armor. The military screen has an option called "view this unit" which you select for that dwarf. Make sure that in the unit view screen, his unit is marked as "on duty". Also, make sure that in that screen, he's set to "unit harasses dangerous wild animals", this will make him kill monkeys and deer as well as goblins and kobolds.
Once you have a drafted, armed and armored, on-duty soldier, you have to tell him where to go. I think the command for this is "x" to access the military placement screen. Once you're in that screen, you can station him at a single point by pressing "s", or you can set up a patrol route for him by pressing "p" in a few places. Now, your dwarf should be kicking ass.
one of my armorers or something got "possessed by unkown forces" or somesuch, they caused her to stop working and make her blink with an exclamation mark. after so long she went crazy, so i locked her in her bedroom. she's brooding in their something aweful, she's torn off her shoe and when i examine it it says that it's 'blood spattered' or something. whats wrong with her and how do i make her stop?
Look up "strange mood" on the DF wiki. If a dwarf can't complete a strange mood, they go crazy and will either explode in a berserk fury like a disgruntled postal worker, or else they will mope around and eventually commit suicide.
finally, dwarves keep leaving the vicinity of my walls to get a drink upstream or downstream the river. the only bad thing about this is that their is a river running straight through the fort which i've zoned as a "drinking spot"! they are risking themselves needlessly, and i want them to stop undermining my paranoid fantasy of security. how do i do so?
As I said, stop making your dwarves drink water. This IS going to present a problem when you get a force of goblin ambushers, which should happen for you within a game year or so. Get yourself a booze industry up and running. Trying to make a nonalcoholic fortress is an advanced challenge reserved for seasoned veterans who think a triple-aquifer Terrifying glacier is too easy.
I hope some of this helps you.