You are correct about the catacombs leading to the next act.
The part of the teleporter you need can be found on the teleporter island itself.
You are on the right track with the mirror, though you only have to check out one of the locations. <span class='standouttext'>Spoiler : </span><span class='spoiler'>Drop the mirror on the ground beside one of the displayed locations, then go use the seeing stone.</span> (highlight the text to read)
