I believe you are mostly right after my tests. The only thing I would like to point out is that the wav file is actually created imediately after you change the ringtone (it seems that changing the ringtone either on the settings or using dbus will trigger a background process that will convert the file into a wav file)
The big chalenge is that you dont have how to know about the wav file converted and stored on the "cache" folder. The dbus get_value call will return the original file location but the phone will use the cached file.
It seems that you just have to assume there will be a file on the cache folder and guess the name based on the "official" file returned by dbus.
Like you I haven't tested yet what happens if you assign a wav file. I hope it will copy the file to the cache folder as well....