Anyone in your team who has access to your repository can approve your pull request.
To know who has access to your pull request, simply search for your team on this page. Your team name should be the same or similar to your repository name.
No, you can't. The purpose of having at least 1 approver for a pull request is to have another pair of eyes reviewing your work and making sure that the changes you made are accurate. Having you approve your own pull request will defeat this purpose.
No. the Isomer team will not be able to help you approve your pull request. Pull requests should be done by content owners of the site simply because you are familiar with your content changes. Having an external party to approve for the sake of publishing is the same as approving your own pull request as the external party is not the content owner, therefore is unable to verify the changes are accurate.
Unfortunately no. This is a limitation imposed by Github, and hence we are unable to change this feature.