I just wanted to pop in and clarify a couple of things for you. First, you said:
"Though already I see a discrepency - the "secret room" card states you can enter for free, but in the preview itself, it states you must pay to enter."
But remember, the preview says:
"Once the secret room entrance token has been placed, the hero who placed it has the option of immediately entering. Thereafter, the cost of entering a secret room (referred to as “exploring”) is one movement point."
So, when the card is first flipped, the hero gets a freebie. After that, he must pay to enter or exit.
Next, you said:
"Also, the fact that the Overlord can remove a challenge token at the start of his turn kinds of spoils the secret room for the heroes doesn't it?"
On the contrary, it helps the heroes! All the heroes have to do is clear the secret room of challenge tokens. Even if the overlord does this for them (by removing challenge tokens), they still reap the rewards.
So why would the overlord do such a thing? Generally, he wouldn't. He'd want the heroes wasting as much time as possible in the secret room. But there are conceivable scenario-based situations in which the overlord may be willing to concede the secret room, if it means sending the heroes back to the main map that much quicker. After all, monsters can't enter the secret room…and we wouldn't want the heroes treating it as a sanctuary, would we?
Thanks for reading!