What happened to Fifi, Roop and Charlie?

At the point in the hospital, Max has become disillusioned by law. He is told to take a vacation, which he does. From that point on, the story is about Max and vigilantism. There's no need to mention any of the characters before that point, as the story doesn't involve them. Yes, it could have been interesting to have them reappear later in the story, but that's not where the scriptwriter chose to go. Instead, we see Max commit violent acts of retribution, and that must have been a wise decision by the writer because IIRC that movie holds (or held) the Guiness Book of World Records record for Highest Profit Margin. It cost less than $500K to make, and has grossed well over $100M.

Possibly a cut scene http://madmaxmovies.com/mad-max/mad-max-cut-scenes/index.html

The bikers return to MFP HQ

Continuing on their rampage of destruction, the Toecutter's gang apparently returns to Main Force Patrol Headquarters on the attack, injuring or killing several officers. This has been reported to me as a scene that was shown in some early prints of Mad Max within the US, and has not been seen since - any further information would be greatly appreciated (contact me). A script excerpt matching this scene was unable to be located.