1. the structures doesn't help with spectators but add to the merchandising costs.
2. the number of spectators depend on prices, performance, type of game and opponent.
3. there is a soft or hard cap on spectators of 20/21k total spectators. This means that you should never attempt to hit the full stadium when you have 7000 lower tier, then it will affect your bleachers.
4. my approach is like this, only look at attendance of league games, figure out by trial and error what the optimal price is on luxury and courtside seats (this is when you are selling out 99% of the seats). Do the same for lower tier. Then see if you sell 13,000 k of bleachers at max price, my guess is yes. Then there is not alot more you can do.
5. also the PR manager has an effect, but I think you should focus on the above.