Is it possible that the gray area that exists on whether a president can even BE indicted is playing a part here?
Technically speaking? Yes. Since Day One, I have been of the opinion that Mueller would never seek to indict Trump, because he is a by-the-books guy, and that would go against DOJ official policy. Even if he did seek an indictment, and Rosenstein or Barr said no (based on the same policy), then Barr would have had to include a reference to that in his notification to Congress today. So it's very much possible that Mueller found evidence that Trump committed a crime, and declined to prosecute simply based on long-standing DOJ policy.
That said, there is no DOJ policy preventing indictments of Jr., Jared, Ivanka, etc. I think it's unlikely (with a few exceptions) that Trump committed any provable criminal offenses on his own, without the help of, especially, Jr. and Jared. Those few exceptions, of course, would include, say, obstruction charges related to the Comey firing. So it's not out of the realm of possibility. But I think -
and I cannot stress enough that this is barely half a step above bald speculation - that it's quite possible, perhaps likely, that Mueller's investigation simply didn't uncover provable criminal conduct by the President.
I know a lot of Democrats will be disappointed to see Trump remain in the Oval. And, although they are quieter about it, quite a few Republicans are probably feeling a twinge of regret at missing out on President Pence. But the idea that this investigation would topple Trump was always - and remains - a longshot. Americans still have the best and least problematic option available for undoing the horrid mistake they made in 2016, and that option becomes available on November 3, 2020.