HI Buff!
Yeah, you're probably right about the stadium size. There have been moments of great excitement and interest, but a big stadium won't create it like a solid program and even average facilities would. 30k could be light since a larger stadium can also provide seating for international events and concerts, but you're probably right. Student support is tough with the commuter campus and majority of students coming in without any interest in the team or legacy. A good rail system to the stadium could be helpful, and the student section would be good for everyone to drive some energy into the crowd. When I went to the last Buffs-Warriors game it was pathetic. The only people with energy were a group of douchebags (one I went to high school with) yelling obscenities at the Buffs players. (We were all only a few rows behind them since the stadium was pretty much empty.)
I don't know any numbers for Manoa, but I have a hard time seeing them make it a viable spot for a football stadium. Your ideas are good, but you're right about the traffic. It's bad enough day to day, let alone during a football game.
Of course UH will never be in the Pac-12. I wish that was a possibility, but they're light years away from that, and much closer to losing the program. I do think there's a world where UH could exist there, but so many thing would need to come together. Funding, support, facilities, stadium, and probably some other intangibles that, as far as I know, no one is working on.