Hi,
I'm a user and I'm unable to help with any leads towards getting in touch with the ox-taskjuggler dev community, but I use org-mode and taskjuggler consistently and faced similar issues to you. I'll present my workarounds in the hope that it may help you temporarily:
1. Try to set your own values for 'org-taskjuggler-valid-resource-attributes' so as to include 'leaves'. Note that the order of words presented is the order in which it is exported. You can do the same for 'task-attributes', 'project-attributes' to further customize export values.
2. To get multiple lines of the same property, I simply type it in the same line as a workaround. So in your case I'd type as follows in org-mode under properties for resources: vacation: "date" vacation "date" vacation "date"
Hope this helps,
Best regards,
KC