(1) If gain/loss is long term gain then Net sale consideration minus Indexed cost of acquisition. If short term then Net sale consideration minus cost of acquisition. Net Sale consideration is sale consideration minus expenses incurred on transfer of property (i.e. brokerage etc.)
(2) Yes you are correct. You can invest in another house property or in bonds. Investment in bonds is limited to Rs. 50 lacs and the time limit is 6 months from the date of transfer of the house property.
(3) In case of purchase: within 2 years from the date of transfer. within 1 year before the sale of the property. In case of construction it is 3 years from the date of sale.
(4) Sale deed, Purchase proof, bank statements, cost of improvements proofs, transfer expenses proofs etc.
(5) Through cost inflation index factor
(6) Tax would be payable in India. For US tax filings contact local CPA.
(7) As the seller is NRI, buyer is liable to deduct TDS@20.8% (excluding surcharge) on the sale consideration. For lower/NIL deduction you have to apply for lower deduction/NIL TDS certificate. For remittance outside India, Form15CA/CB have to be filed.
For detailed discussion you may opt for phone consultation.