Sequential numbering for pseudo-"Rows"
Using way more math than should be needed, there is a formula that works. In the dummy spreadsheet above, you would put this in A3 and then copy it into all subsequent rows in bulk, just as you'd expect:

1 + ((    SQRT(8*SUM(  OFFSET($A$2,0,0,SUM(ROW(),-2),1)  )+1)    - 1) ÷ 2)