Hey Andrew,
I am sorry for the delay of this reply,
1) Having some error #200 when trying to edit / update a post.
The reason why you hitting this error is most likely because you haven't submit this user permission "publish_action" for Facebook review, so when this user trying to save the post, it do not have permission to autopost to their Facebook profile page.
To temporary fix this, you can disable this setting from
backend > Easyblog > autopost > facebook > Allow Authors To Use Their Facebook Account - NO Screenshot :
http://screencast.com/t/LwJUfKziUTiM
Then follow the documentation here how to submit for review.
->
http://stackideas.com/docs/easyblog/administrators/autoposting/setting-up-facebook-app
2) Embedding YouTube in the blog - original 3.9 blog post video does not work.
Do you mean when you editing your existing post which created from Easyblog 3.9? If yes, can you provide us which post are you mention that so I can help you take a look on this.
3) Composer not showing, instead JCE although it is set to use Composer.
I suspect one of the possibility is the user profile already set JCE editor by default so it will not show composer when you create blog post that time. Check my screenshot :
http://screencast.com/t/le3RczMrg9
Hope this helps.