I believe in one of the booklets that came with the boxsets it was mentioned that they tried to work off of an opposite colour scheme as well. And the style of architecture is the exact opposite of the effect they were trying to achieve in series!Ohtori, hard and angled lines as opposed to soft and curved lines. I think the main purpose was for a combination of reasons, a) to let us know right away that this place is a whole other beast to the school we knew from the series, that place was a little strange and metaphorical, but this place is an acid trip don't expect anything to make sense b) the setting is inverted, not just different, like Utena and Anthy are I might be way off, but I always felt like the inverting of Ohtori was also signal to the inverting of U&A c) With the success of the series, there seemed to be an attitude of, "Hey, we can do anything we want now without a whole bunch of people breathing down our necks, lets get crazy!" So, yes some of it probably was "because we can"

Added note on the Akio car, why would he need the backseat when he didn't have series!Akio swag, it's not like he would be using it

The whole television show tower being uprooted and tacked onto the movie one always creeped me out. But it makes sense, since the movie literally rips Akio's penis right off.

As for the smoother designs, my understanding of that is that Hasegawa designs in those two different ways based either on the impact he wants to have or the practicality of budget. It's harder to animate the smooth designs of the movie.

But yeah I never noticed his car had no backseat. That would make it a better match to the actual model of car it's based on, but with fewer pimportunities.