I have done a considerable ampount of alexa integration but not much with chromecast or other media players such as plex even though I have both in my home.
The main issue is the way alexa integration currently works with OH2 is not reqally suitable for an ything other than switches, temperture and lighting.
I have a couple of small exceptions to this in the media space I did integrate a ‘Mute’ rule
in this case I need to say "Alexa Turn on mute’ and she then sends the ON command to the switch item in OH, this runs a rule that sends a buttonpress on my harmony hub to my TV that then sends the IR signal that mutes the TV.
I have a similar setup for un-muting too.
So the first issue for me was the clumsy way to request the mute function. The family and guests would naturally want to say such phrases as "Alexa, mute the TV’.
so to answer your questions
1a. Is it possible to control the audio (played from raspberry to chromecast)
YES
1b. via alexa (“play next song”. “pause music”, etc)?
Not in a intuitive way
As for question 2, I am not 100% clear to me what you are actually saying perhaps further clarity is needed.
Thank you for sharing your experiences, much appreciated!
Let me clarify my 2nd question:
Let’s assume I have a Sonos speaker, which is connected to Alexa and OH2. Sonos is playing a song from raspberry pi, after it got triggered by a rule. While the song is still playing, I say “Alexa, play x song from spotify”. So the Sonos would have to switch from playing stuff from raspberry, to playing stuff from Alexa.