Well, just a point of clarification...I don't think
Turin was in The
Hobbit. It was
Thorin and company who took 'the burglar' on his grand adventure, with a little assistance (one might even say a bit of a nudge) from
Gandalf.
Yes, Gandalf had to look carefully for all the markers to get them where they were trying to go, because
Rivendell is indeed called "the secret valley of Rivendell".
There's no doubt
Elrond's power (and the power of his ring) had some things to do with how it remained so carefully guarded and preserved.
But in all my reading so far, I don't think I've come across anything telling us how Rivendell was first located and established by Elrond, other than that he was forced to go north by
Sauron's
armies and ultimately established it as a
fortress against Sauron in 1697
Second Age.
So I have to set this question..."did
Ulmo assist" for short...squarely in the Great
Halls of Speculation. I've just got no facts.
But it would seem to me it's more likely a 'happy accident' in the course of battle for
Eriador at the time.
Perhaps some of our real lore-masters can shed light.