(a) how large an initial expansion of the arena should be?
arena building don't cost money besides a fixed amount of the seats, so even a fast building of two "VIP places" could be a sensefl upgrade, and would be probadly my first upgrade pretty from start on.
(b) if i'm in a position to expand my arena? and if not when?
yes, if you have fund you don't want to pay for players the next time. With the current system i think it is good to start early with upgrading your arena, because you make more money with more selled seats and less prices.
But if you need a backup you also could raise prices instead, i tried in my carrier not to go about in the last quarter of the price scale, so for exmple sell my bleachers below 16$. Also i tried to arrange the prices the way, that i have a few free seats in this category, because else i would be afraid that masses of visitors don't get the ticket they want. So i would raise the prices for courtside and Bleachers in your example, and i believe your reduction in the lower TIE is to big when you doesn't expand to 600 seats.
(c) what are the recommended proportions for the different sections of the arena?
because of the flexible prices there is no optional proportion in the arena. For teams who aren't stong enough to win there away games, i think it makes more sense to expand the expensiv places(VIP, Courtside) first, for teams who ussually have a great fan survey because they often win the away match the cheap places are very lucrative.
IMPORTANT: In the next season we have less visitors then today, but you could sell your seats in your arena for the original price if you have to much of it.