This all assumes that there will be two chains, which will only happen if the fork is done without consensus.
I have a lot of doubts.
First, my BTC before the fork can be translated to XT any time (if its my will). I have a BTC mining contract, after fork BTC mined cannot get converted to XT, am I right?
It depends on what chain your mining contract is mining on, but yes, any BTC mined after the fork cannot be exchanged between the two chains unless an exchange is willing to do it.
What happen if I buy BTC on a exchange after fork, I'll be able to convert it to XT?
Only if another exchange is willing to exchange it.
What would happen if I dont like XT or XT crash, may I come back to BTC?
Yes, if you did not touch the BTC chain after the fork. If your Bitcoin was only transacted on the XT fork, then if you decide to use the old BTC chain, you will have however many BTC you had prior to the fork.
It is possible a market manipulation? Example: A group of guys make buying pressure to pump BTC-XT, they convince a lot of people to change to XT because have best prices, then dump the price and run out?
Yup. Its called a pump and dump. Happens all the time with new altcoins.