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.

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First the program will compute the correct depreciation when you enter the required info in the asset section of the Sch E section  ... and second you must amend the 2017 return to correct this error.   If this property was rented out only 1/2 the year and you converted it from personal use then read the screens carefully AND review the return carefully as well. 

https://ttlc.intuit.com/replies/6603004