Mugi - blackmailing

I think you missed the point of the haggling scene. Knowing Mugi, she probably had no idea that she's get the price dropped so much just because her father owned the store, but that isn't even the point. It served as characterization for Mugi copying Ritsu and wanting to be like a normal high school girl which can be seen in the Ritsu/Mugi episode much more in season 2 as well as episode two where she copies Ritsu by dumping her fries into the same pile. Actually, that is the biggest part of Mugi's character- she just wants to act like a normal girl.

Ritsu, Sawako - abusive

Sure, you could say that she was abusive to Mio as a child, but that was mostly out of just being a total idiot and not realizing how embarrassed and shy she gets. At least in the end they become best friends because Ritsu is actually a good-natured person.

And Sawako... while I wouldn't say she is abusive, you could say that her forcing of outfits onto the characters is questionable. It makes sense in the context of her personality and serves as an outlet for her since acting like the perfect teacher can lead to the building up of stress. She's dumb and irresponsible like Ritsu, but they grow so much by the end of the show that you'll love them as people.

Mio, Azusa - bossy pieces of shit

I don't really understand how you came to that conclusion. They are both very hard workers and want the best for their friends, and of course they'd get pissed off if the rest of the group was dicking around. They are strict on Yui and Ritsu because those two lovable idiots would otherwise be worthless members of society if there was nobody to keep them in line. They never boss the others around in ruthless ways and are visibly act lovingly to the others.