1) Upgrading to First class via the kiosk will more than likely do nothing to help you toward your Gold challenge. You only earn AAdvantage miles and elite-qualifying points (which you need for the challenge) according to the original fare class you purchase. Paying for an upgrade does not change that.

2) As for how AA decides whether to offer the upgrades -- probably a mystery, but I'd guess that if there are more available FC seats than elites who had requested the upgrade, they'll offer up those seats for purchase at the kiosk. I would say that if you get an ExpertFlyer subscription, you can search for "X" inventory, which is what the elite upgrades book into. If, less than 24 hours before the flight, there is X inventory still available, it probably means all elites (including Golds) who requested upgrades have received them, and thus AA will probably be looking to generate some extra revenue from the kiosk upgrades. They'll still probably leave a few seats for any elite who changes his/her mind at the last minute and wants an upgrade and for anyone who switches flights and wants an upgrade, but if you see something like X3 or X4, I'd be willing to bet AA will offer at least 1 or 2 kiosk upgrades.

The 6:31am flight DFW-PHL is showing F7 and X4, after the Platinum upgrade window has opened. Given that it's an early Sunday morning flight, probably not a lot of business travelers with status looking for upgrades. Unlikely there are 7 Golds who requested upgrades. FWIW, there's only one exit row pre-assigned - that's sometimes a very very rough clue as to how many elites may be on a plane.

But again, upgrading won't help you any toward your challenge. And keep in mind that the upgrade on this flight will probably run you $150, last we heard here, it was $50 for every 500 miles, and this is a 1300 mile flight. Pricing may have changed slightly (which might explain your $90 fee), but it'll certainly cost more than your last upgrade.