A) There can be only one, and there should have been.. All I have to say about Highlander 2..
B) Mad Max was the first movie, so a sequel would fall between that and Road Warrior. However, the trailer is ambiguous, he still has the interceptor (the last interceptor), but it is destroyed, so it is impossible to tell where it falls in the series. The end of RW saw the interceptor blown up where it is just rolled in this one, so maybe at the end he goes back and patches it up... ? Either way, you can't go from Mel to unknown actor to Mel.. it is too much of a disconnect.
It's kind of like trying to follow the Jack Ryan movies where there were 3 or 4 different Jack Ryans.. While each story is independent of the others, they still have some common history (and a friggin name ffs), so at least stick with a similar actor and not vary it by putting drastically different actors (with drastically different ages) in the same role..
At least with movies you can get away with it though.. IMHO, in a book series it is blasphemy to do that. I was a huge fan of W.E.B. Griffin until he went from one book to the next in his Badge of Honor series and jumped ahead 2 decades but left the characters the same age, just "updated" them. One book he was using pay phones and CB radios and the next they are using cell phones and laptop computers. But all the history and events are referred to as if they were mere weeks before. No excuse for doing that in a book whatsoever.. I surmised from that debacle that the author had gone senile and was no longer worth reading. I gave him one more shot after that but his next book was just as bad - butchered the past books and the history of the series in question. ruined it for me, so I stopped reading his stuff.
With movies I just don't really care much..