Thanks for this long break down of all characters, their positions, and why you think they're in those spots. It was really interesting to read. :)
Elly is currently the character with the least CGs of all main girls, and no deeper storyline about herself yet. I hope that'll improve when more events about her will get added.
In general, all main characters will receive more events in the future. Maybe some of the current problems of characters will be solved with those events.
There wasn't much content planned for Brad and Trey originally, but I started to add more for them recently and will continue to do so in the future, and look at them more as main characters rather than side characters.