Hi - the reason the button requires full trust is because it is accessing the local file system. Basically, it extracts the form template files so that it can use the schema file.
There isn't going to be a work around with the current version of the template, I'm afraid - we should probably keep this in mind as a future improvement.
You can hand create your mapping file, if you need to. You'll need to know how to get your SharePoint list GUID, and the internal column names. You'll also need to know the XPaths to your form nodes. I can get you a sample mapping if this sounds like an option for you.