I believe Mr Darcy to be the former friend who helps, and do not believe he required help from others within the ____shire Militia or elsewhere. And since I believe a commission required more money than influence, Darcy was rich and powerful enough in his own right to undertake this himself.

And I think you are right in referring to Aunt Gardiner's statement regarding the negotations for Lydia's marriage. I think it seems pointless on Darcy's side to negotiate a marriage settlement that does not provide a way for Wickham to 'earn' some money, however small, in addition to the 10,000 pounds we learn of through Mr Bennet. If Darcy was clever (and I don't doubt this for a moment), any capital settled on Lydia would be tied up as best he could so Wickham could not get at it, with any income associated with the regulars going straight to Wickham. But this of course is speculation.