Q: Is the supplied HDMI cable a 2.1 variant that can handle EARC?
Of course it is. Why would Sonos ship a cable that is not compatible with the Arc?
Q: I have noticed that playing music sometimes no sound comes out of the surrounds.
This depends on your TV settings and source material. Look for settings called “passthrough” so the TV does not change anything on the source material - but if that's not 5.1 Sonos will not use your surrounds (much)..
Q: I also want to use someone's iPhone to do the Sonos tuning.
This is possible.
@Henmen Have you tried adjusting Music Playback to Full instead of Ambient under the Surround Audio settings?
Sorry. The surrounds not playing is not with TV but with music. This answer by @GuitarSuperstar is better - mine was focussed on TV sound. Not music from Sonos sources.
I tried full that is much better thanks.