Best answer by melvimbe
There really isn't a need to connect your do the play 5 via line in anymore. You can use Alexa groups (in the Alexa app) to specify that the Play 5 is the preferred speaker for the dot.
As for the Spotify issue, have you setup the Spotify account within the Sonos app? The account needs to be setup in Alexa and Sonos to work properly. It sounds like you did so, but the scenario your describing makes it sound like it's not.
When you say that you're using the iphone to start spotify, which app are you using? Sonos, Spotify, Alexa? Are you using airplay?
Have you enabled the Sonos smartskill? If not you definitely need to do that.