A few comments,
The 1650 is 16 channels, not 16 zones. It will give you 8 stereo zones.
I think you'd be ok without a preamp between echo dots and the amp. Each zone has gain controls that should allow you to trim output so "Alexa volume 10" is not damaging to speakers, distorting, deafening or whatever.
The 1650 is stable to 4 ohms as long as you aren't bridging channels into that load. Choose speakers that are 8 ohm and you can parallel two on each channel. You could look for 4 ohms and run in serial, but choices are limited and IMO wiring is more difficult in ceiling applications especially later when you can't see where everything is going. My suggestion is if you want 3 speakers in a zone, one pair should be Left Right and the third a single speaker stereo (dual voice coil) so that impedance stays balanced.