We are creating a presentation in Spanish and have found inconsistent support of HTML/special characters in Pachyderm. So far we've had the best luck using the ASCII codes found on this website:
http://www.starr.net/is/type/htmlcodes.html
Note that the ASCII codes are different than the HTML codes, but can still be displayed properly by a web browser. For instance, ö and ö are equivalent and will display as ö.
The results have not been perfect. For the Main body text fields, the special characters support seems to be consistent as long as you use the ASCII code and not try to paste the character itself. For the subtitle fields, as you mentioned, it's definitely hit or miss. We've found that the Slider template does support HTML, while the Dual Comparison template does not.
Also, we would love to see a summary of your usability testing report if it's complete. Thank you.