1. If the cancellation deed has been done by a registered deed then it is valid.
The recital in the cancellation deed confirms that the buyer has no claims agaisnt the seller in this regard, moreover if he has given a receipt for the amount acknowledging the receipt for the same duly witnessed, that shall be more than sufficient.
2. Since both the buyer and the seller have jointly executed the registered cancellation deed cancelling the registered sale deed executed by the seller in favor of the buyer, any claim that may be made by the buyer shall be with the seller alone and not on the subsequent buyer.
Further, since the buyer has acknowledged the receipt of the return of the sale consideration amount, the buyer cannot claim any amount at a later stage and even if he does, it would not be maintainable in law.
3. Not necessary, and it is not possible to cancel the cancellation deed and again execute a cancellation deed, If necessary you may ask hem to execute a registered rectification to rectify the error i.e., to include the left out words i.e., the return of the sale consideration amount.