then got the raft stuck on the dock where I first left and now can't use it.
Er, which side? There are three sides you can leave the raft on: Lower, upper right by the chest, and upper left. There's at least one illusionary wall by the chest. (To its right, I believe.)

I then made the sylph by the medallion thing visible, but it didn't help in me being able to get past it.
I didn't get past it either. Would love some tips.

To get the key:
Spoiler: show

Also, when you need to get the key, there's a little square lake one screen north of the solitary fairy fountain in the room with the western locked door. Somewhere on the right side of that lake, (On the water.) you can smoke/burn an invisible Slyph. The fairy will be roving in a square path above the lake. Use the Sylph there to grab the key from midair. It's tough, but it does work if you're tenacious.