The overworld sprites will change based on rank yes! Though I can only guarantee a change at rank 3 and 5 currently since it would be a lot of work.
Sorry about the mushroom tunnels thing, that's a bug I wasn't aware of... I'll make sure to fix this in the next version.
You won't be able to get back inside normally, so for now you'll need to use debug mode to get back inside:
Hold Enter and Left shift, then press F2 and you'll hear a jingle indicating that debug mode has been activated.
Then, press Left control to enable the ability to walk through any door (you'll need to leave and come back to the current room for it to take effect).
I recommend pressing Left control again once you're done to disable that mode, and then doing the same key combo as before to disable debug mode (Hold Enter and Left shift, then press F2).
I also recommend saving in Biscotti so this doesn't happen again!