Well, we still can't edit job notifications via SQLDeveloper. However in case anyone has a similar problem in the future, here's some PL/SQL I'm using to fix them:
DBMS_SCHEDULER.REMOVE_JOB_EMAIL_NOTIFICATION (job_name => '<job name>'); DBMS_SCHEDULER.ADD_JOB_EMAIL_NOTIFICATION ( job_name => '<job name>', recipients => '<required recipients>', sender => '<sender>', subject => 'Oracle Scheduler Job Notification '||sys.database_name||'@'||sys_context('USERENV','SERVER_HOST')||' - %job_owner%.%job_name%.%job_subname% %event_type%', events => 'JOB_BROKEN,JOB_CHAIN_STALLED,JOB_DISABLED,JOB_FAILED,JOB_OVER_MAX_DUR,JOB_SCH_LIM_REACHED' );
(you can omit the events parameter if you're happy with the default list in the documentation.)
It's kind of a blessing in disguise I suppose because it was the final kick I needed to get around to scripting the [re] creation of scheduler jobs so they can be added to source control.