This will prevent Mark from sending you messages, friend request or from viewing your profile. They will not be notified. Comments on discussion boards from them will be hidden by default.
My objection to the slippery slope argument is that everything is a slippery slope until it isn’t.
The option to share highlights to Goodreads doesn’t always appear. It seems when I click on highlights, then scroll up and down, it often appears then. I’m using Kindle app on iOS now.