Data selectors for custom fields
In the rules config UI I can't see any "Add Loop" button - is this to be expected in version D8 Alpha 3?
  


yes ... as per the issue about Implement Loop UI.


  
  What data selector should I use for accessing field_items on the Order node that has just been saved?
  


I doubt this is possible in the D8-alfa3 release of Rules. Though you may want to retry using the D8-dev release ...


  
  What data selector should I use in the rules component to access field_reservation_status on the current object from field_items?
  


Refer to my answer to "2." above ...


  
  How do I set up a loop over the variable that is created in the reacting rule?
  


Refer to my answer to "1." above ...

PS: No doubt you'll run into more-of-the-same issues using any of the currently available releases of Rules for D8. However, using the D8-dev release is currently your best bet/alternative.