Dear Concerned,
A. No as there is no agreement about brokerage services and payment it will be difficult to prove the financial relationship - more over s you have already paid the money - you can always say that the money decided has already been paid.
B. Yes messages sent to the broker will be used against you if the case is filed.
c. Thats your choice - you may not need to reply the notice - depending on what amount is due to the Broker - the broker may decide to file a case - for eg if the commission paid is upto say 1-2 lacs - broker may not file a case a the lawyer fee itself would be decent - however if it is more than 2 lacs he may consider filing - for that matter a case just to bring you to criminal record in India would be enough for him to harass you.
d. Depends on the whats app conversation - if say you have agreed to pay 4 lac rupees over messages and you have paid 2 lacs - you may have to pay the balance as you have already paid 2 proving the transaction and the balance because you agreed.
WHAT"S the way out - You may choose to reply to this person's notice, through your lawyer here in India and based on review of all the discussions between yourself and the broker - tune the reply in a manner that you safeguard yourself from any liability.
You may choose to send the entire communication and whats app message chat between yourself and your broker for a Free Review of the documents (on my email id) and suggest you for next steps.
Best of Luck