1) I agree with someone's comment about leading indicator...there isn't one really one. I suppose those who trade stocks can and should use futures as the indicator. The logic behind it is easy to understand. If you have an understanding of how index arb. works, you will know why it is wise to monitor the activities of futures b/c futures has the power to influence the indexes (thereby the stocks within those indexes). Sometimes, futures will lead the indexes. Hence, at this time, it could be used as a leading indicator. Most of the time, indexes will perform just as well as futures. At this time, futures is no longer a leading indicator. This takes me to my next point about market manipulation.
2) I suppose it is possible to manipulate the market short-term. This is could be done by trading instruments like futures (high leverage to boost buying power). Because futures can often have an effect on stock indexes (from index arb.) as I have mentioned earlier. If you're a trader who monitors the price movement of futures carefully, you will see what's going on. The market can be manipulated to profit from those who don't monitor price movement carefully. It is also possible for you to profit from this market manipulation by spotting and riding the wave.
In theory, in order to manipulate the market that is trending up, one would have to sell so much to cause a reversal (sell more than people are buying). This reversal could be within a short or long time frame. If it is short, it should exibit certain anomolies which technical analysis could pick up as a sudden reversal. If it is of longer time frame, technical analysis could see that as a down trend. By using indicators, you are protected from losing alot in events of market manipulation. Everything is reflected in the price. You can choose to go in and ride the wave, or sit out if it's too dangerous. Since, futures are electronically traded, there is no designated specialist or MM to manipulate the price, unlike stocks. By following price movment of futures, you could be profitable. Just come up with an effective way to filter out market noise (which is often harder said then done).
3) I don't understand why it is so hard to make money trading the market? It's easy, really! First, you search for someone who consistently loses money. Then, you do the opposite of what he does. When he sells, you buy; when he buys, you sell. You will be consistently profitable, very profitable.