Thanks for taking the time to give a full answer.
I don't live in Ottawa, I've spent most of my life there and am aware of the various parts of the city and where the arena is to be located. I'm also aware of all of the LTR talk as well. Kanata makes up a large part of the fanbase, and while I agree that a downtownish arena would be more accessible in general, I'm not sure folks should be going ahead an pencilling in larger crowds by default.
Winnipeg may be smaller population-wise, but more of them buy tickets, they pay more for their tickets, and they sell out their building. Ottawa, a much larger city as you point out, sells fewer tickets, has cheaper tickets, and can't sell out regularly, including key games in the playoffs.
I'm not sure that 18000 or less isn't leaving any money on the table, it's creating some scarcity the proper way (not tarping) and allows Ottawa's relatively small fanbase to sell out they building regularly.
Personally I look at a fanbase that has not really been able to sell out the arena consistently without thousands of free tickets, and I'm not sure why you'd want to build an arena that is too big for the fan base. At the moment it's EM, before it's mediocre teams, before that it's... There will be another excuse coming for why fans opt not to buy tickets and we'll continue to see thousands of empty seats. We don't have all of the corporate support that other markets do.
Anyways, do we know what the proposed arena seats?