I am looking at adding some Sonos products to enhance the music/radio playing capability in my home.

Some questions:

1. Is it possible to use Alexa voice assistant on an Amazon Echo to direct to play (music or radio station) simultaneously (and in sync) on a Sonos Amp (with 4 speakers) and a Sonos 5?

Yes.

1. Can I have it set up so I can just say “Alexa Play Song X”, and it defaults to playing on the Sonos Amp and Sonos 5 together and in sync. (I understand that it cannot play in sync on the Echo and the Sonos devices, so I would not want the music to play on the Echo when the Sonos devices are playing.)

Yes again.

1. How would I set this up? Do I set up a Sonos Group? Alexa Group? Or Both? What would be the “Preferred Speaker” or “Controlling Speaker”, etc?

An Alexa Group - set the echo as the ‘enabling’/‘controlling’ device for the group and set the Amp and Five as the preferred speakers for playback. Also add both to the second section of the group so they ‘duck’ their audio when speaking to the echo device.

1. Can I ask Alexa (via the Echo) to adjust the volume of play on these Sonos devices simultaneously? (e.g., “Alexa, Louder”.)  

Yes, or set a volume level, such as 4 or 40%.

1. What if for preferred listening I would like the volume of the Amp to be louder and that of the Sonos 5 to be softer, or vice versa?  How would I make such adjustments via Alexa

Just include the Sonos room name in your instruction to set the volume level.

1. If set up this way and then, for a change, I want the Echo to play on its own speaker only, can I ask it to do that? How would that be done?

Yes, just use the name of the echo device in your playback instruction.

I am new to this. Appreciate your support. Thanks.