So, yes, The Sandman is part of the DCU. John Constantine and Madame Xanadu are part of the DCU. Swamp Thing is part of the DCU.
...
As for the Tim Hunter question, I can't really answer that.
I believe Tim crossed over with all of those characters and more (including Etrigan, Zatarra and Spectre).
Yep. The first Books of Magic is (however entertaining) very little more than a guided tour of DC's magic community. The last time I checked, Tim Hunter had nothing whatsoever to do with the DCU, however. Kind of a shame, since that was his whole point as originally conceived. But I don't really know too much about Tim Hunter, although I'm rather certain Day of Vengeance owes a lot to Mr. E's showcase of the future seen in Books of Magic.
It's really hard to square John Constantine with the actual DCU, too, even though at one point he was very much ingrained within it.
As noted above, Sandman was very well-established as part of the DCU--but it has very strange limits on when it can be invoked as part of the canon, owing to Neil Gaiman's dead hand control over the franchise.
So, you have Dream still very occasionally appearing in DCU books, and Death was in Action Comics quite recently. You'd think she might have given more of a shit about the Black Lanterns, but then you'd also think she'd try to get her brother out of the cage he'd been stuck in for seventy years, so there you go.
Does anyone know what the actual deal with Gaiman and DC regarding Death is? Is it contractual, or merely gentlemanly brand-maintenance? It's
probably for the best, but I don't see why Gaiman gets to lord it over Death till the day he meets her, when he was free to do anything
he wanted with other people's creations (Destiny, Element Girl, the Fury, Kirby's Sandman, Doctor Destiny, and so on).