First off welcome to the group! Livecycle can be an interesting program.
For me I am not very technically proficient in coding so I tend to gravitate to, how you say, less than elegant solutions. What I do is create an Email Submit button for each Radio button. Then I create a dummy Email Submit button (this part will be key later). From the Object Library view on each Email Submit button I set the Email Address and Email Subject that I want. Then I hide all of the Email Submit buttons with the exception of the dummy Email Submit button. From the Action Builder I then create an action on the dummy Email Submit that when the button is clicked to open a dialog box to say "Please Select an Option". I then create an action within the Action Builder on each Radio button to show the corresponding Email Submit button that has the subject line I need while also making sure to hide the dummy Email Submit button (and any other buttons if the user clicks several radio buttons). That worked for me in the past. Then I added a DocuReady event on the dummy Email Submit button to make sure it was visible and the other buttons were hidden once the form was open. Sort of a reset if you will. The downside is if the user changes nothing they still get a prompt to save the changes. I am sure that some of our more advanced coders can offer a fix to your code lines, but that is sort of the non-coder way to go about this.
Edit: Oh I forgot to say that if you want the buttons to appear the same to the user, just overlay them together. To the user there is no change.