the spotless catering contract is basically Locked in for eternity for the stadium. Not much that can be done about it.
I think they've come up with quite clever ways to come up with work around alternatives to the expensive chip and hotdog options that spotless offer. As bwtcf mentions above, those options are a reasonable price and good quality. They've also done well at working around beer sponsorship in pushing for craft beer.
I also doubt they're deliberately pricing teams out of the stadium. They have their own maintenance/upkeep/project costs too, and they need revenue from tenants to keep up with all of that as they get no funding from council. No doubt they'll be working with the Phoenix to get this resolved as some money from the nix is better than no nix at all.
I do think more can be done between the nix, stadium and council to get more people to games. The council (and their public transport company) in particular seem to be very meh about getting people to events, and have very little communication between them - a prime example being no trains running during several events, bus drivers not knowing about free trips for season pass holders etc.
As far as I can tell, the stadium did used to have the same communication problem with the nix, but I think communication between the stadium and the nix has improved remarkably in recent years. You're right that increasing parking prices does seem to go against bringing people to the stadium, and it would be interesting to hear an explanation for it. Maybe they needed to recoup costs, maybe they're trying to encourage public transport use? Or encourage people to park there earlier to take advantage of the discount?