Tala says it's hidden when she hacks the computers. That's what draw her attention to it. Obi-Wan was presumably guided by the Force. Or possibly Tala, it's not made clear how much she is doing to help him along his way.
Yeah, Tala's assistance seems mostly limited to opening the underwater door then providing Obi-Wan with a distraction so he can free Leia. He seems to find his way to Leia (via the "trophy" room) on his own. I suppose Tala also gave him the code stick thing that let him access that restricted area too. (I was actually a bit surprised that her officer clearance was good enough to access that area, to be honest.)
Vader is the biggest drama queen in the series. Almost everything he does from the word go is (badly) trying to manage his traumas and seek a sense of control over his life. I can fully imagine him keeping a trophy room full of frozen/dead Jedi as a way to assert himself over that aspect of his past. And repeatedly exposing yourself to your traumas to keep the pain fresh and emotions powerful seems like a very Sith thing to do.
While I agree, I think if the Jedi corpses are anyone's trophies, they're the inquisitors', not Vader's. Fortress Inquisitorius isn't Vader's castle, even if they do look similar. I would think he would keep his personal trophies closer to home, in his own basement, not his minions' basement. (The father of one of my childhood friends was a bow hunter, and he kept his taxidermied trophies in the basement, out of sight from polite company. We used to call it the basement of death.)