Point 1- the Gophers Indoor Football Practice Field has 90' clear to the center peak. You can get away with about 85' or so, but not much lower than that (assuming you have a sloped structure). If you don't need to punt, you can be down at about 55-60' clear at the highest point. Add up 30' tall of building materials all the way around a 100,000+sf building, and it's a substantial cost. It's merely a cost vs. benefit issue, nothing more.
Point 2- yes, the Gophers still have the Nagurski indoor facility that is now used by all the other sports. It is a bit of a disaster, in that they deferred maintenance on it for so long, anticipating the new facility would go where it was located (and thus would be demolished). That didn't happen, and they had to put money into it to just make it habitable (insulation was falling down from the roof, the roof was leaking like a sieve, etc.). I don't know what Northwestern had before, but I'm guessing they didn't have something like this, so they had to double-dip and have football play nice and share with the other sports.