Hi Giuseppe. Most of your questions are answered here http://mebfaber.com/timing-model/
Shorting is more complicated than one thinks. It is more like insurance, you are more likely to pay for it before you get anything back, and if. From a statistical point of view, the market always goes up, and the distribution is biased that way. Shorting means you go against that bias.
The only way I was able to define shorting is that it is a function of time(like gambling), if red came 5 times in a row, then a black is more likely to come. Therefore as time goes by, with nothing notable, the probability that an event will trigger a selloff increases (i.e housing burst, many fossil fuel companies go under at the same time could be another). But again, even if 5 reds came in line the probability that the next one is red, still 50%. Oil went down to $26 but then recovered and no harm done, at least not yet!
You may want to test your theory using /posts/a-simple-downside-protection-model
you can easily substitute the bond with SH and the securities with SPY and observe results
That's just my two pennies' worth