Hi Arun,
To start with, as the sale agreement is executed on May 17 , this would be forming part of your AY 2018-19 (PY 2017-18). Due date for the same as per section 139 is July 31 (hope you are filing the tax return in the capacity of individual).
Coming to the 3 core questions raised by you ,
1) You can park this amount in your S/B account till the due date as per section 139 -July 31 2018.
2) In principle, you can open a Capital Gain Deposit account before the afore said due date. However it is safe to open the same a month or two ahead to ensure error free documentation. - say April 30 2018. Accordingly the date in pt.1 above would be April 2018.
3) Proof of Deposit needs to be documented and furnished whenever demanded by the department. The amount in this account cannot be used for any purpose other than the designated investment. Hence any application of funds from this account for any other purposes would be charged to tax.