Hi Steve,
Is the first error happening with any and all attachments? I haven't seen the error ("Invalid character in a Base-64 string") before, so I want to first rule out the possibility that it may be dependent on a specific attachment.
For the second error, how did you launch the form when you get the error? There are two possibilities: (1) You preview the form from your machine. This, in my experience, leads to permission errors. (2) You publish the form out to Sharepoint and open it from there.
Can you post a separate question (under Tools) for the InfoPath to Sharepoint List tool? I can't promise a quick reply for that, but it'll at least get on our radars. Thanks.