On Kite, the price will depend on demand and supply. On Coin, the amount blocked for G-secs is the estimated weighted average rate of all allotments to competitive bidders + accrued interest + markup value (difference between the lowest and highest bid).
The excess amount that remains after reducing the allotment amount from the blocked amount will be credited back to your trading account. More on this here: On what basis does Zerodha block money for the bids for G-secs?
Yes, you will be able to pledge since the G-Secs will be credited to your demat account upon allotment.