I made a mistake... misread what "half a year" referred to in Darcy's letter.

The two accounts about fathers' deaths are:
Wickham's: (Ch. 16) ..."Mr. Darcy often acknowledged himself to be under the greatest obligations to my father's active superintendance, and when, immediately before my father's death, Mr. Darcy gave him a voluntary promise of providing for me, I am convinced that he felt it to be as much a debt of gratitude to him as of affection to myself."

and

Darcy's: (Ch. 35) "His own father did not long survive mine, and within half a year from these events, Mr. Wickham wrote to inform me that, having finally resolved against taking orders, he hoped I should not think it unreasonable for him to expect some more immediate pecuniary advantage, in lieu of the preferment, by which he could not be benefited."