Regarding "failure gives some resource that can be used later" being gamed ("Bill and Ted play twenty rounds of darts"), the first thing that comes to mind is requiring the failure and the later use to be part of the same story. The heroes suffer setbacks in Act 1 and especially Act 2, only to triumph in Act 3. (Hopefully your plot-runner sticks around long enough for there to be an Act 3.) Or at least require them to be roughly on the same level. Or place a cap on them, which should also help guard against hoarding (if you're already at e.g. 5/5 then you may as well go down to 4/5 to get out of a minor jam).
Waiting forever in massive scenes is a good reason to not have massive scenes, or at least not have them all in the same place. Break that fifteen-person group into three five-person groups working on a different part of the problem, then bring them together again for a big after-party where they can share war stories.
As for spotlight hogs, yeah, you need to have some opportunities for a scene where they're not around. The plot-runner can exclude them by OOC fiat (PCs not approved by me are elsewhere and unable to arrive in time), or PCs with the potential to hog the spotlight may voluntarily stick to "you guys need the field experience, I'll be available, call me in as the cavalry in case things go majorly south". Or, as above, just break them up into two or more groups, where the big PCs deal with big things while the little PCs deal with little things.