Surely a universe, being simply a collection of copyrights, is also part of the deceased's estate and subject to the same rules? If so Geek of Ages could apply to them for permission.
OK, as suggested elsewhere those same executors or administrators may not even know that the deceased did write stories and might be confused - their problem!
Alas, a major problem here on SOL is that many authors either never admit what they write on the site, or their families simply don't care about preserving their heritage.
If the family wished to preserve their reputation, then surely they'd sign in at least in the seven years following his death. Even if they didn't have his password readily available, they could easily reach out to our great leader, possibly include a copy of the death certificate, and request a new password for the account.
Instead, most families either don't know about the authors' activities, or consider it a 'dirty little secret never to be mentioned, less everyone discover what a pervert grandpa was'. :(
If those families ever bothered examining his email, they'd realize the fan base the author had, which tells us they've probably never even glanced at them—meaning they're not about to notice any polite requests for permissions.
As far as the law is concerned—at least without a well-funded publisher with millions of dollars in potential losses at stake—your best bet is the ol' 'it's better to ask forgiveness after the fact than permission before it's done'. That's especially true because I seriously doubt that any author on SOL has ever filed a copyright application, meaning there's NOTHING to be achieved by bringing a copyright lawsuit to court, (no financial loss=nothing to recover), regardless of how they might feel about it.
In short, you've already got permission from the author to post your story. It's in everyone's best interests if you do, and there are NO negatives to doing so.