I'm a little confused about where the extension currently lives.
As you mention, you have a self publisher by default. If the extension is in the list for this publisher, only you can reply to reviews.
Alternatively, you can move the extension to a group publisher. Once you've done this, any members of that group publisher who are registered as developers and have paid the $5 fee will be able to reply.
It isn't possible that your extension is in the group publisher and the group publisher has been deleted, as if the group publisher was deleted the extension would no longer be live.
I hope that helps.