Yes:

Administrators and staffing users can approve a job on behalf of someone: 

• In Recruiting, go to a job's approval record and click the person's name in the Approval Bar. See this article for the complete process.
  

Yes:

Administrators and staffing users can skip the approval process after a request has been submitted:

• In Recruiting during the Select an Approval Process prompt, click Skip Approval. See this article for the complete process.

Yes, after a job is approved:

• Administrators and staffing users can edit anything about the job.
• Standard users can edit only a job description, and not other information.

If you are a Standard user and are not selected as an approver for the job, it is hidden from you.

For security reasons, only Administrators and staffing users can see all the jobs in the Approval process.

No: 

We have a feature called ECHO, where if you reply #Approve or #Deny to the approval request email, Recruiting automatically adjusts the job approval process accordingly. 

For more information, see How Do I Use Echo with Requests?.