It was not supporting HTML NEWSLETTER_DEFAULT_FCOMCODE templates (welcome mails use that too, but newsletters are supposed to allow it to be in either HTML or Comcode format). I have fixed that.
You may want to remove the salutation and footer from your welcome mails, given the template also includes this.
Oh. and if you want to test, I left some test code in your data_custom/execute_temp.php, just change the email from mine to yours, and it will send all welcome emails to you for you to preview.
You may want to remove the salutation and footer from your welcome mails, given the template also includes this.