Once a case record has been created, the script goes off and does some other stuff and based on this info I may need to set the custevent_has_job_sent - however, it seems I can no longer use _CASE. Without loading the newly created record, is there another way to set the field?

you can use record.submitFields, but your old record is done after you save it

so _CASE.submitFields ?

record.submitFields(options)