since cs is your second choice therefore you will need a lot of luck for that to happen
if you are allotted ec then you wont be able to get back any of your lower options
yes if you are allotted cs then you will have to be present there to cancel your higher options
you have a good rank and you will easily get any of your options
during real time allotments i dont think you will be able to stop your further upgradation unless you give everything in writing and sign on the register maintained at the councelling venue