Snape's power is severely curtailed with regards to Harry, and wisely, because of their mutual acrimony, but he still has a greater measure of power over Harry (especially now that the secret is out) because he is still Harry’s teacher.  Once again, he can give Harry detention and make his life difficult, mark him unfairly, and use his position in other unfair and cruel ways.

I have had unfair and cruel teachers.  I have a crippling illness, and I used to have to go to the principal to overrule those teachers when they used it as an excuse to limit my education.  Yet they still had more power than I did, because they still held the grade book, and I still had to go to their classes and listen to them when they derided me.  Harry has a similar situation with Snape.  Because of Snape’s bias, Harry has more leeway than most students, and he uses it to fight back, but Snape still holds more power.

I got the impression from the seventh book that the students of Slytherin house had used the year of Snape as headmaster to lord it over the other students instead of fighting.  Their actions spoke for themselves.  McGonagall and Dumbledore are undoubtedly biased, and Harry has more power and fewer consequences to his disrespect than most students, but Snape is still his teacher.

Belby is neither a Gryffindor nor a favorite of McGonagall, nor is she enabling him for that reason.  She wants to fire him in a way that doesn’t raise his suspicions (difficult) and then wait for him to go back to his fraudulent ways so that she can send in Magical Law Enforcement and they can gather evidence with a clear chain of possession.  She’s right.  In front of the Wizangamot, the notebooks would be made irrelevant because it was the “untrustworthy” Severus Snape who discovered them.  She also has enough flair to wait for him to have another “big discovery” and then discredit him right after. 

Having known people very much indeed like the Marauders and the Weasley twins, and having been both target of their bullying and object of their affections (I got pretty in high school, and all of a sudden, boys who had been shoving me in trash cans for years wanted me to date them.  It was so wonderful to tell them to piss off) I can tell you, that they are intelligent, thoughtless, charismatic, and charming, or they wouldn’t have the sort of power they have.  Most of them are good people underneath the idiocy, and most grow up to show it.  My brothers are two such people, and they grew up to start thinking and recognizing that they’re doing something wrong, and some actually seek to make amends.  I know you don’t see it that way, but students shouldn’t be expelled for what the Marauders did, with the exception of Sirius and the Shrieking Shack.  I’m sure they spent a lot of time in rather disgusting detentions, though.  Also, some of the blame should be laid at Dumbledore’s door.  He’s the one who believed in second chances most strongly, and I agree with him.

Harry actually has realized what he did, and talked about it with Hermione in an earlier chapter.  He gets it shoved into his face a little bit later on, but he has thought of it.  Remember, though, he had no choice.  He either left Snape and Eileen with an abusive parent, or he risked ripping time and space apart.  Not a great choice.