2: if the administrator of my vps, or a hacker hacked in my vps, he got the access of all my files, he saw my source code with the password, then he can steal all my coins with the password right? any other ways to improve the protection of such accidents? (because there are always new vulnerables found, for apache, for linux etc..)
Yes, the only way the prevent it is with multi-sig transactions, which is coming soon.
3: how can i know about the translation fee before send bitcoins? ie, if a client has 15btcs in his account, and he send 15btcs out, how can i know how much is the fee of this translation before process it? because it might be free, it might be 0.01btc, it might be 1btc, i need to calculate the amount available before process his request. btw, i don't really get the definition of translation fee, is that obligatory or voluntary? if you pay, your request get faster process by btc network,if not, your request will still be processed, but just slower, right?
You can set it to 0, but if the transaction is too "spammy", the client will still send it with the fee.