3- is it true that people hold their merit and only give it to their alt accounts and their friends or trade it ? that mean for a new comer like me that its quite difficult if not saying impossible 
The other questions can best be answered by reading, and they'll probably get answered here anyway. As far as this one goes, I suspect it's true. I've seen merits being sold for something like $9 to $19, and that's probably in the ballpark for what they're going for. It's crazy, but after being here over 3 years and seeing what gets sold on bitcointalk, I am not surprised in the least. So some people are hoarding merits to sell on the black/gray market. Definitely. Then there are inactive accounts where merit is just sitting there forever.
I've seen quite a few members who I suspect are trading or selling merits, but the sending of merits is extremely subjective and I don't feel comfortable handing out negative trust when I have doubts--even if the doubts are very small. Don't know what the statistics are, but I suspect that most of the individuals who are using the merit system "wrong" are sending it back and forth between friends and/or alt accounts.