So here is what I did:
added feed
Updating it and click on the *Topic Title*.
After this I clicked the top menu, then on Enclosures, then on the url and "Open url externally" the browser opens and ask to play with media player or to download. Clicked download and it is working, also media player is working. The button "Open URL in browser" is not working.

It seems I can't follow you what you did. I don't know how to use this url openers or if the custom download command really works.
The url is without ssl encryption so no problem for the browser.
Maybe you need to write an enclosure plugin. Don't know how the application works.

I did the same with the other podcast feed it has a ssl url but it is working too with this method.

edit: My test were done with the latest version from git and qssl correction and user-agent fix for app and client which hopefully solves all download problems. Released under cutenews 1.3.2