1) Is it possible to use this ability without an expose marker?
Yes. It's not an requirement.
2) Is it possible to use the heroism part of the ability without having an expose marker?
Yes as well.
3) since the card does not say "push to separate distant group" like some other cards, does it mean the characters are pushed out in group? Or are they all separate.
As the basic rule for pushing, always push each character to separate group.
4) One question not relating to the above: In zombie encounters, does using dodge when a zombie attacks allow the hero to get pushed?
Yes. Only attacking zombies can't push characters, Ranger is pushing himself with and ability card.