Sale agreement of flat

1. If the sale agreement is not executed by a registered document then it is not enforceable therefore you can very well take action for cancelling the sale agreement and claim full refund, failing which you can file a money recovery suit against the seller.
2. Yes
3. Court will not go by the mind of the buyer or the seller, the court will appreciate the evidences alone before it. 
4. The seller has given an acknowledgment for the receipt of money hence he cannot deny the liability.

a) if sale agreement is notarized, can buyer still invoke cancellation clause within the agreement ?

Yes, the buyer can decide either to invoke the clauses of the agreement or revoke the sale agreement depending on his circumstances 

To my knowledge, the agreement for sale is enforceable even in if it is registered unless the State Government, through the appropriate provision of law has made such registration mandatory. 

In case the prospective buyer rescinds the agreement, the other party is bound to return the advance less the loss, if any, he has suffered and the terms of the agreement.

To mention the date of refund in agreement does not arise, and fifteen days notice point comes whe seller decided to cancel the agreement

In my above posting, "even in if it is registered" may kindly be read as "even in if it is not registered".

Lapse is regretted.

Notarised agreement is enforcable.

Show the document to a local lawyer for analyses and proper advise

 does enforceable means that buyer can invoke the cancellation clause 

and send cancellation notice to seller ?

if seller refuses to accept cancellation, which court buyer can go for refund?

my question is refund and not cancellation.

legally, is refund supposed to be executed as per agreement ot seller can charge extra outside scope of agreement??

Refund of amount paid by buyer without cancellation / revocation of the agreement is a hypothetical situation.

Seek advise / guidance of your teacher / professor /tutor / coach.