#1 you are correct, your best option right now would be to force it to the bottom of the articledisplay.ascx file, though you do run the risk of upgrade issues.
#2 The only way you could do this at this point would be to setup a specific page for each article that needs a gallery module on it, and setup that page as the "display page" for that particular article.
#3 Currently there isn't a way to have an image included without just putting it into the HTML of the article itself.
We're looking at a token system for a future version of Publish that might be a step closer to providing this type of functionality.