For cards that have the word "or" example... if failed that investigator must either take 1 damage and 1 horror or resolve the hunter keyword on each enemy in play. If there is no hunters in play do we have to take the damage and horror?

For whatever reason, this is under "Must" in the rules reference.

If an investigator is instructed that he or she “must” choose among multiple options, the investigator is compelled to choose an option that has the potential to change the game state.

Choosing the hunter option will have no effect if there are no hunters, so you are not allowed to choose that option. You must take the damage and horror.

Thanks as another example the card mass hysteria says you must either take 2 damage or do something with other cards in play but if they are not in play then you have to choose the take 2 damage.

is this available yet? I'm in UK, UK stores dont seem to have it just core set and rougarou

Also falls under the Grim Rule...

Also falls under the Grim Rule...

The Grim Rule does not apply at all. jaqenZann quotes the correct rule which means there is no ambiguity, which means there is no Grim Rule application.

For whatever reason, this is under "Must" in the rules reference.

If an investigator is instructed that he or she “must” choose among multiple options, the investigator is compelled to choose an option that has the potential to change the game state.

Choosing the hunter option will have no effect if there are no hunters, so you are not allowed to choose that option. You must take the damage and horror.