The number of congregations can increase because of double counting in the same way the WT can double-count publishers.Congregation B and C are merged creating another congregation + 1.
4 congregations + 1 new congregation = 5 congregations.
No. The number of congregations are counted on the basis of the numbers of congregations who submitted their Yearly 2016 Congregation Analysis Report.
So in your example there were not 5, but only 3 congregations counted for the stats as in September 2017, as only 3 congregations submitted their Cong Analysis Report