When did the law change to disqualify the gain that was accrued during the rental period? Before 2018, I believe. 

In other words, the gain accrued from 2005 through 2017, plus depreciation recapture, could never be excluded. The entire gain, not including depreciation, must be allocated by month. In your scenario, 13 years from 2005 through 2017 were non qualified. 

4 years are qualified,  so only 4 out of 17 years of gain could be excluded. 

13 out of 17 years of gain, plus recapture, are non excluded.