As far the multiple hubs, I did located a support article on the Logitech site. The article stated that multiple Harmony hubs cannot be used with Alexa, and that even though there are 3rd party solutions available, they won't allow for all functionality. The Logitech work around was to deactivate hubs and activate the one you want to use that moment. Seems rather kluge to me!

Hubs are required because the Alexa devices don't have the built in radios to talk directly to most devices. Some like the TP-Link bulbs and sockets have built the capability directly into their devices at the cost of bulky devices, but most devices still require hubs. As a radio person I know that too many signals can cause many issues, so I can envision scenarios wherein multiple hubs transmitting to multiple devices could blow Alexa's little mind and cause all kinds of bad device control behavior/issues.