You're running up against several common forms issues. Some background may be helpful:

1) Message forms published to a public folder will not be useful. Outlook will be able to find the form and show it only if users put messages into that public folder. For a custom message form to be useful, it needs to be published to the Organizational Forms library, not a public folder.

2) Because message forms create items that will be sent, not saved in a folder, you cannot make a message form the default form for a folder. This is something of a moot point, though, because of #1.

3) The solution, if you want to use a public folder as a forms library, is to publish the forms to Org Forms, as in #1, and then use a special Post form in the public folder to act as a form launcher. The code behind the launcher form creates a new item from the designated published form. Thus, each form will appear to the user as an item in the folder, and opening the item will launch a new instance of the desired form. See http://www.outlookcode.com/d/forms/formlauncher.htm to get the form launcher form.