Did the group ever had admins?
Unfortunately, there isn't a way to make you an admin since there was an admin at one point within the group. The only option that you have is to create a new group and add the members so that you can be the Admin and continue the group conversation.
If you can take a minute and use the rating system to rate our session as EXCELLENT, GOOD, or OK and press Submit so I am credited with helping you.
The only way to delete the group is to have admin rights or someone with admins rights can delete it. If you have reported it to Facebook,it is up to them to determine if the group can be deleted. It can take weeks for their replies though because they do not offer live support since they are a free service.
Unfortunately, you are looking for a solution that does not exist even though you remain hopeful, it will not change the answer.