This is a serious flaw in your integration with Amazon Echo. I want my Echos (I have 3) to be family devices but want to listen to music on my phone when family members are playing music at home. I will be switching to Pandora permanently and canceling my Spotify premium is this isn't fixed ASAP.
A Spotify account is intended for personal use, as it allows to stream on one device at a time. Have you checked Premium for Family? You can add up to 5 sub-accounts and each one of them will have the opportunity to choose their own music. We'll make sure to pass your feedback about this too.