@deborahh To your question 1: If you block a whole instance, it should filter all accounts on that instance. I you block only one account, only this account will be filtered.
And you can choose how to post. "Followers only" makes your posts only visible to followers, "unlisted" also to others (ex: by boosts) and "public" brings you even into the Live Feeds of your instance and other instances that are federated with your instance.