I think Stringer was being portrayed as trying to be something he wasn't and couldn't be. Such a great character, like with the copy store it's showing that if Stringer had wanted to, he could have operated a legit small business. That is what he could have done, but he tried to fly to high, too ambitious.
That speech where he tells Avon that they don't have to be out fighting for corners anymore, that they can be above it all. To us, the audience, it seems so reasonable and logical, like of course, who wouldn't want that. But Avon responds 'I want my corners.' and that he's 'just a gangsta', I suppose.' because it shows that Avon understands his own limitations.
At first we think that it's tragic he still 'wants his corners' but we realize that it's just him being honest with himself. He's not a 'suit wearin' mothafucka' (iirc).
I think Bell is just constantly shown as not really fitting in anywhere, not really fit to be a criminal, and not really fit to be a big player with the city deals. The only place he really does fit is, as you say, when he's in school and with the copy store.
He was trying to be something he wasn't.