A. As per the General rule of law, Agreement for Sale need not be registered unless possession delivered. Hence, by paying sufficient stamp duty as prescribed in the law is sufficient. Hence, your presence is immaterial if the registration is not requried.
B. Your main objective of the query is whether the execution of agreement by way of aforesaid manner is valid or not? Yes absolutely vaild subject to payment of sufficient stamp duty either franking or purchasing e stamp paper and non delivery of possession. And the builder or authorized person must be signed in all the pages. Moreover, if you don't have any issue related to Title of the land, builder reputation, terms and conditions of the agreement, mode of payment towards the consideration you can issue post dated cheque. However, PAN card is mandatory only at the time of registration of Sale Deed if your sale consideration amount exceeds more than 50 Lakh for the purpose of TDS. Else not mandatory in most of the sub registrar office. Hence, If the seller had been signed over the agreement in favor of yourself is sufficient record in the eyes of the law.
C. Notwithstaning anything contained in the A and B reply, you can execute Special Power of Attorney within the family member to complete this task if you are not satisfied with these procedures.