I tried to explain it in my original post when I proposed the idea that a like-for-like replacement essentially indicates that you are replacing a player whose primary role remains the same i.e. opening batsman for opening batsman; middle order batsman for middle order batsman, keeper for keeper, spinner for spinner, fast bowler for fast bowler.
In the case of all-rounders, there is nothing theoretically wrong with batsmen or bowlers being replaced by all-rounders as long as the primary role of those all-rounders remains the same. I gave the example of Hussain being replaced by Kallis, as their primary roles were the same and the fact that Kallis could bowl was mere bonus. On the other hand, Hussain couldn't be replaced by Imran or Botham as they were bowlers first. There is a bit of a grey area for all-rounders who are not specialists in either category and I think we can allow that to pass.
In answer to your questions:
1. Depends on the players being replaced: theoretically Daniel Vettori is an all-rounder but his primary role is a spinner. I can't replace Neil Johnson in my team with Vettori as that would change the composition/structure of my side. On the safe side, I would advocate no to this question.
2. Yes, shouldn't be a problem as long as the all-rounder fast bowler is a bowler first, batsman second.
3. See above.