Subject: how to prevent editing of rt field by user
You might want to look at a weblog post I wrote a while back about this exact problem. The post was called Rocky’s Workaround, and it suggests a couple of ways to handle this.
Attachments are a challenge even with computed rich text. You can take the approach of saving the attachment elsewhere and simply referring to it. I am not sure whether you can prevent opening it using any technique if it is on the document somewhere.