1. Check you haven't relocated the database/form since originally setting up the main (or otherwise) data connection.
When you publish a form, it still expects to find the Access database where you originally connected to it. You can 'repoint' the form's main datasource via the main dataconnection configuration.
2. You can submit via email until the right part of the workflow where the database submit is needed - only make the submit to db button available then.
Though if retrieving records back into the form are part of your requirement you'll still need the db somewhere that all users who read/submit to it can access it via the form.
3. The form in an email is already just 'the data' - but if you want the submitted data to be in excel format or otherwise, you'll probably need to have some code to convert the form xml to something else and then manually create email/attach and send. i.e. not really a built in InfoPath function.