As you already mentioned, the rules state you may use the Quick Cast Marker either before or after you take a Creature Action. This means that all of the examples you provided are acceptable. To break it down into the simplest terms, the following are acceptable:
1) Use your Quick Cast Marker, fully resolve whatever spell you cast, and then Activate a creature and fully resolve the creature's action.
2) Activate a creature and fully resolve the creature's action, and then flip the Quick Cast Marker and fully resolve the spell cast.
Both of those options are legal and are performed before you pass turn to your opponent. What is not legal, however, is trying to use the Quick Cast Marker in the middle of a Creature Action. Once you flip a marker, be it Quick Cast Marker or Action Marker, you must fully resolve everything you want to do with it before you can move on.
For Example, the following are NOT legal to perform.
1) Activate a Creature, Move into another zone, Quick Cast a Spell, and then perform a Quick Attack with the Creature.
2) Attack with a Double Strike and use a Quick Cast in between the two strikes.