If you’re in the US all new construction should have neutrals in the switch boxes per code
That is called a 3-way. You can use a dimmer and a dumb on/off, dimmer and an aux switch, or two dimmers (this is an advanced configuration but easy to do via associations). All configurations will require you set parameters on the dimmer or via the hub.
The dimmers are the standard “decora” or rectangle cut out.
They work fine offline. You will not able to be able to trigger scenes (ie tap 2x UP to turn dim to 50%. Dimming will still work by holding the paddle.) Associations will also work offline.
Switch connected to a dimmable bulb is fine. There is a bulb compatibility document. I’ll find the link.
The product page talks about Over-The-Air updates. How is that possible without an internet connection? Are updates a manual process that I initiate from the hub? Are there risks involved in updating the software for something like this (where bugs affect home operation)?
The process varies by hub. For example in home assistant I download a firmware update to my phone. Open the HA app go to zwavejs2mqtt find the dimmer and upload the firmware.
There are several of us on the forum that use zwavejs2mqtt. I don’t have any issues. Is there a particular issue you’re worried about?