If anyone is curious I think I managed to sort this out. I was overthinking it - using a glass node gave me both the shininess I wanted and the translucency so I didn't need the confusing mix of nodes I was originally trying to use.

Now I just need to finish modeling the slimey tentacle monster it's meant to go on. I made a rough version in Sculptris Alpha but I want to try and create it entirely in Blender, so I'm off to watch some videos on dynamic topology sculpting. :)