Hi, i am testing your Plugin as well with Wordpress Version 4.9.1 The posting on Steem wont work (both with new posts, and updated posts) i always get the message:

Please wait for 300 seconds to be able to publish this post to the Steem blockchain as there is a 5 minute cooldown.

I waited long enough. Could this be an issue with other plugins like YOST or maybe Spellchecking?
I would love to use your plugin for my blog!