if you have not agreed with the salary, of course
you can just tell HR that you want to further discuss about your salary.

come in
可以说考虑几天,然后把合约拿回去研究研究

this is possible and one of my friends tried it out, but later he decided to reject the offer so he used register mail to mail the contract back.

for ur 1st question, there is nothing is impossible in this world, so you can still try, but of course according to common sense, you success rate 49%, not 50%. But still, why not try if you are really intend to do it?:)

Good Luck

Strongly suggest u negotiate it.
unless u desperately want a job
or
u like the job very much and the offer is within reasonable range