key: 1. Indispensable, supremely important. 2. Important, salient.
https://en.wiktionary.org/wiki/key#Adjective
keystone: 1. (architecture) The top stone of an arch. 2. Something on which other things depend for support.
https://en.wiktionary.org/wiki/keystone
I grew up in the "keystone state". As far as I am concerned these words mean precisely the same thing in this context, and neither has stronger implications than the other. Are you sure you're not the one spinning here?