@Larry_Post re alexa control of next track.
If your remote is setup as suggested, then in extension settings, options section see Seek/Skip track function and select reversed dual buttons.
Then you can use Alexa Fast Forward or Alexa ask Harmony to press fast forward depending on the Alexa harmony skill in use.
Variation of the obvious next, next track, skip next etc all seem to result in somewhat unhelpful responses and/or questions from Alexa 
Alexa Pause, Alexa Play Alexa Mute, Alexa Volume Up, Alexa Volume Down all seem to work fine. I would expect ask harmony to Press 0…9 type command to work OK for playlist/radio selection as well.
I have tried dot get the Roku emulation to expose list/radio names to Harmony, but they were ignored.