1) Can we survive (in the long-term) without water?
Yes, but your wounded will die. It'll be like living with hemophilia in a world with warfare and without modern medicine. Anyway, place a well over the murky pool; they'll give that to the wounded.
2) If I ever succeed in funding the Aquifer, will it also be salt water?
Yes.
3) If I can find a cavern level with pools, will they be salt water, too?
No. The water in the caverns of a saltwater map is usually clean.
4) Is the water in the cavern pools limited, or does it replenish?
Depends. If the water is at the edge of your map, it should replenish. Or, if you're unlucky, you could get one of those freak maps where the liquids actually leak out of the edges of the map to the outside. (I once had that happen on a multi-z-level magma sea)
If the caverns' pools do not touch the edges of the map, then it won't replenish. No rain in the caverns, you see.
5) I'm told that salt water can be desalinated by pumping it into a reservoir made entirely out of constructions (floor, walls and ceiling)... is this correct?
Yes, if you insist that your wounded shall have clean water, then construct a 1x2ish little room onto your surface, with a pump taking from a murky pool. Put a windmill on it or power it with dwarven power. The water in that tiny reservoir will be clean, unless you start to pour it somewhere else.
Same goes for your aquifers, when you find them. Dig a bigger room in the z-level above the aquifer, construct the pump, poke a hole from the z-level above the reservoir into the ceiling of the latter, fill the reservoir, and you have a nice big reservoir of clean water.
You can also go at the water resource from the z-levels below and have a reservoir of clean water in the depths. Just dig a tunnel from below towards the aquifer, construct the floors of it all to prevent tree growth that would block your plumbing, install a depressurizer at the bottom, a pump and a reservoir (bonus points if you make it automatized). Finally, dig a ramp into the aquifer. The miner should escape with his life (if you're lucky and he's fast), and your reservoir is tidy, wonderful and full of clean water.
What you need to be aware though is that the water will "contaminate" as soon as you have it fall through a "salty" z-level, i.e. a z-level that contains natural salty water.
Also, draining a reservoir and then refilling it in the same way can somehow "contaminate" the water. That happened once to me and I still don't know why, except that I had drained and refilled the thing.
6) I'm told that Dwarves will drink salt water out of a Well. Is this true?
Yes. No known health issues for the dwarves are known to me at this point.
7) If yes, will they give this salt water (from the Well) to wounded Dwarves?
Yes.