I don't think there is any evidence for the stance at all. A cup by the painter Makron shows a guy readying a long sling-- . The Aspendian tetradrachms show a guy reading his sling by holding the pouch before his eyes, but that can develop any way. Virgil and Vegetius mention three turns above the head as normal, so what's called "helicopter" here, but was that normal earlier ?

Some kind of spin, and some kind of throw, sideways or vertically, is most likely.

Make sure that your slinger never shoots in a straight line ("whanging"), but always L-O-B-S his shot (great big heavy stones) with incredible force and accuracy. You might include a sidekick who has just started slinging but is terrible at it, flatlines but always misses because he over-stresses his shots.

Even better, have your slingers ARGUE about this while peltasts run them down and capture them and hoplites nail them to trees.

A Greek slinger of that period would use lead sling bullets weighing about one ounce. He would sling further than the archers of that period (see Xenophon's Anabasis) at a rate of ten shots a minute. A skirmishing group of sixty slingers (a small number for those days) would have a rate of fire equivalent to a machinegun.

They would use speed and rough ground to protect themselves from infantry attacks. Their greatest vulnerability was to cavalry. 

Slings can crack very loudly, or just make a soft 'whup', as desired. The projectiles hum or buzz in flight due to their very rapid rotation. On impact they would smash bones and pierce unprotected flesh but plate armour was enough to stop them. Seasoned Hoplites could take a sling bombardment but any lesser units without armour or discipline would be cut down and routed.

This is an excellent example of what we call the 'Greek style' today: http://www.youtube.com/watch?v=YTjrg6gwrsA

We have no idea how the Greeks actually used to sling because it was never described, but this takes their starting position and is very fast and powerful.

Here are some loud cracking slings with heavy stones from Mallorca: http://www.youtube.com/watch?v=F4VwbJ8f7bE

ignore thearos on the lobbing - in a battle situation with consistent made ammo, an experienced slinger will pretty much sling in a flat trajectory up to about 100 - 150 metres.

I can keep up that kind of slinging all day, and have on consequetive days
Thearos can't and seems to ignore anything he can't personally do in this regard

The thing about slinging is that the sling is simply an extension of your arm. Stronger, fitter, trained arms are far more capable of sustaining flat trajectory slinging than untrained arms.
If you guy was a reguar slinger than you should assume that he's capable of sustained flat trajectory target slinging at leats out to 100 metres. Which is a lot further than most people realise.

It's a common trait among historians, with a few notable exceptions, to ignore anything they can't personally do. 

Lobbing is more for clearing obstacles like walls, ditches etc. Also useful for slinging larger rocks over walls, houses. In a riot situation you could lob large rocks into the next street quite easily.

For long distance battle slinging with moulded lead glandes, you would sling in a shallow parabola - not lobbing as such. A good slinger would be able to make 250-350 metres consistently. With lead ammo (that's important)

The common injuries sustained while slinging are very similiar to american baseball players.
Generally elbows, and shoulders.
Again if you sling regularly these parts of your body will be stronger and less prone to injury. Also fitness work and targeted exercise will massively reduce risk of injury.

Style wise - yeah, there's no real evidence. The depicted slingers on pottery-ware are always depicted in the same manner. But that starting position can lead to many different styles of slinging.

As far as sidearm being more effective - again it depends on what you are doing. Personaly I use it for distance slinging and fig 8 for closer more accurate slinging.

The problem with slings in ancient greece is that they were primarily peasant weapons and most historians didn't write much about peasants. So there's very little detail on greek slinging and slingers.

What we can do is show you how ANY slinger works and fights with a sling. Given the way slings work it's unlikely that different countries had radically different styles and techniques.

One thing I can tell you from personal experience is the sound a lead glande makes coming straight for you.
1 single glande sounds like a swarm of bees, I had one land about 10 feet directly in front of me - never saw it -but bloody hell was it loud !
I suspect this is a unique experience in the modern era

The other people with me were hiding behind a quad bike about 30 feet away - my sanity has always been questionable

Anyways, notice that in the Anabasis, the Rhodians say that they can sling lead-- which does suppose that it's not a widespread skill in 401 BC. Troops of the satrap Tissaphernes MAY have adopted it shortly afterwards, which again suggests that it's a novelty.

Livy, using Polybius, seems to indicate that Achaian slingers practice on beaches (of course) by shooting through rings.

Note that a very important part of slinging is the windup.