1) As you are not having any taxable income, there was no need to file IT Return in the earlier years. There is also no need to file IT Return u/s 139(3) of the Income Tax Act to claim carry forward of loss under the Head Income from House Property.. Hence, you can file a return now for the FY 2017-18 before 31st July 2018 claiming the carry forward of loss for the earlier years also under the Head " Income from House Property".
The interest on your housing loan upto the date of occupation in 2016 should be capitalised and allowed in 5 equal annual installments.
If you have significant taxable income in the FY 2017-18, it is worthwhile taking efforts to compute your total income for the years from 2016 and onwards and claim carry forward of losses for the set off against the income earned during the FY 2017-18.
2) You can claim deduction in respect of repayment of principal amount u/s 80C upto the extent of Rs 1.5 lakhs along with other eligible investments/expenses during the FY 2017-18.
3) You need to link your aadhar, if you have any, with your PAN for the filing of your IT Return. You need to check for any entries in 26AS, i.e., the statement of TDS to claim TDS if any and offer such income also in your IT Return. As you are a non-resident, your income earned outside India is not taxable, but it is advisable to review the incomes earned outside India to confirm that they are not taxable in India. Pl also check the entries in your Bank pass books to confirm that you are offering income properly.
Your other queries in the follow up question:
1) There is no deduction for depreciation on rental income. However, a standard deduction of 30% is allowed on the rental income net of municipal taxes paid, if any. This is allowed even if you have not incurred any expenditure on your let out house property.
2) Interest on loan against the house is sufficient.
3) The end use of the loan is not required to be shown if its a mortgage loan. However, in the case of housing loan, the loan is given for the purpose of acquisition of the house property or improvement. Hence, the end use is automatically ensured.