Probably best to post question in the DH thread here: Roon Extension: Deep Harmony - rich feature set for Logitech Harmony - #641 by Beer_Good
One hub should do, location does not matter, it only needs to be on the same network as the device that will host the extension and your Nucleus.
Yes, you can use the Harmony app and/or the physical remote. I only have one hub and don’t know how easy a remote can switch. I believe Adam has a couple as he’s included the ability to support more than one.
The Harmony mobile app does everything the desktop versions do/did just have to figure out the ‘new’ way. I kept the app installed on a Windows 10 machine for some time but haven’t used it for a year or more since I figured out the mobile app.
None of this will impact the Nucleus nor your ability to play music. It only adds another way to control Roon.
See you over on the DH thread.