If Hoffman conceded based on faulty information provided by the election officies, I think he has a right to a recount or some other remedy.....
Therein lies the issue that most young law school students often struggle - there's frequently a great divide between the way the law should work, and how it does work.
And, it's not the fact that he conceded, which is legally irrelevant, it's the fact that he didn't contest the election, which is what allowed Pelosi to seat Owens, so quickly, in the first place.
Special elections can be problematic for this very reason. In a normal election, there time - usually a couple months - between the election, and the expiration of the incumbent's term. This allows for all manner of election contests to play themselves out. And if they aren't, then you can see events transpire similar to what happened in MN this year. The Senate refused to seat Franken because Coleman formally challenged the election.