Critter covered most of this, but here are the answers to your questions:
(1) You must amend. It can not be added to your 2018 tax return.
(2) Your depreciable Basis is the LOWER of (1) your Adjusted Basis (usually your original purchase price plus cost of improvements), and (2) the Fair Market Value when you converted it to a rental. Then the cost of land is subtracted from that amount. So if your Adjusted Basis is lower than the Fair Market Value, the tax assessor's estimate does not matter.
(3) The first month only gets a half of a month of depreciation. So if it was "placed in service" in July, you will get 5.5 months of depreciation.
(4) Your situation seems to keep changing. Two months ago you said it was rented for 2 months, then you said 3 months, now you are saying 6 months.
