yeah if you use online wallet he only need your password and if you use offline wallet all he need is to hack your pc it is harder than password but there is very skilled hacker that will do the impossible to reach there goals.
The chances that your desktop gets hacked in comparison to a web wallet are much lower. You would have to either: 1) Attract attention somehow; 2) Do something dumb, e.g. click on suspicious link.
Of course it is possible, and that is why all bitcoin wallets are permanently trying to improve their security measures, trying to minimize this risk.
It is not possible to 'hack Bitcoin from your wallet'. This would imply that the Bitcoin software itself is flawed, which it isn't. Hacking the desktop that has Bitcoin on it, is something else (this is possible).
I think using one bitcoin address or same bitcoin address for every bitcoin transactions will not make your bitcoin stolen.
Exactly. There is no correlation with the number of times that you've used an address and the likelihood of getting hacked.