I saw this on ebay for $3.58 and free shipping Mercedes key fob replacement shell. Our key won't quite open all the way with this shell but ! the panic button will fit our key fob and for $3.58 with free shipping it's worth it.

https://www.ebay.com/itm/Folding-Fli....c100167.m2940

 

All I can say is that the replacement key fob I bought for $6 - twice on ebay - was a good replacement for the original single key fob case that came with the car that I bought. I replaced the batteries and have a good quality "shell" for the electronics and flip key. I'd say don't be concerned with the panic button. The whole shell is what you will be investing in.. Look at the feed back and make your purchase accordingly.. If as you say the key doesn't retract fully, look for another vendor. Mine works perfectly in that regard..

 

The only reason I posted this is just for the people whose panic button has cracked, fallen apart or just looks bad and wants to replace it .The key will fully retract but only opens to about 87 degrees instead of 90 degrees .
I bought it just for the panic button aesthetics