One obvious option would be to leave a bit at the bottom space of the screen and place the "next" button there:
<iframe ... style="...bottom: 30px;..."></iframe>
You will need some similar positioning for the next button to appear at the very bottom.
Another option is to automatically use
SoSciTools.submitPage() based on some condition, e.g., after a certain time. You may also place the HTML files in the project directory, which will allow communication between the frames. Usually XSS-prevention of the browsers disallow that.