The Brett series took plenty of liberties, such as adapting the stories out of order, combining some stories into single episodes (for instance combining "The Mazarin Stone" with "The Three Garridebs"), expanding other stories with new material, writing Moriarty into "The Red Headed League," writing Lestrade into "The Creeping Man," writing Mycroft into "The Golden Pince-Nez" in place of Watson and "The Mazarin Stone" in place of Holmes, etc. etc. Heck, just having Mycroft take an active role in an investigation is shockingly out of character, but it was made necessary by Edward Hardwicke's unavailability in the former case and Jeremy Brett's illness in the latter.
I read all sixty Conan Doyle Holmes stories back when the Brett series was in its original run, and I noticed plenty of differences between the two. Heck, Brett's portrayal of Holmes was far more manic and expressive than the cold, saturnine, withdrawn figure described in prose. It did indeed take liberties, just as every adaptation does, just as every adaptation is supposed to. The word "adapt" means to change something to fit a new context or purpose. If all you want is something exactly like the original, just go back to the original itself. The whole point of doing an adaptation is to find a fresh take on the concept, to bring something new and different to it.
And I don't see what's so shocking about Watson punching Holmes. Watson was an English gentleman, which means he was brought up to consider fisticuffs an appropriate way to settle an affair of honor. And Holmes was an antisocial individual, so it's not impossible that he could've done something to offend Watson's honor, especially if it pertained to his fiancee as it did here. Sure, the Watson described in the stories might never have done such a thing, but since those stories were narrated by Dr. Watson himself, it follows that he wouldn't necessarily have been entirely forthcoming about his own less admirable moments, or about personal clashes with Holmes that he would've deemed inappropriate material to broadcast to the readers of The Strand.