LOVE the art! What amazing talent!
This is a really cool concept! I can see you were starting to think about systems that would require you to work both inside and outside the shell -- as is, there isn't a whole lot to do outside the shell aside from make readjustments if the opening is against a wall and the one spin block.
The music and scenario made me feel like there was a sense of urgency, but the gameplay was about slow, fine control. I think what would make this mechanic REALLY sing would be if there were some external tasks that required attention -- you need to defend or maintain something outside your shell, then go spelunking inside a shell, then race back to repair/refortify whatever fell to pieces while you were gone.
The neck mechanics feel so good, especially the retraction sequence. The moments waiting for it to complete are phenomenal, and could be a great source of tension if there were something you were rushing to get to ASAP. Really driving the feeling of being "overextended" if you'll pardon the pun.
This is a really creative take on the theme, with a lot of juice and a core mechanic that just feels really fun to use! Very very nice work!