4. If am selected for a job and I am not available for the first pre-departure briefing, can I attend a later briefing and take this job later?

No, the jobs offered by the Junior Programme are to be taken on within three months after the pre-departure briefing.
When you apply for a job, you actually commit to liberating for the briefing and for the departure period set in the job description.

In case of doubt, you can contact the Junior Programme Project Officer mentioned in the specific job description before applying.