1 You pretty much always want a hub. This is what runs the show and what protect the hardware. Anything without a hub is probably just using one in the cloud where your dependent on them keeping running, to many manufactures have closed things down and left people stranded over the years. If you can not control it completely locally it's simply not yours your renting it from somebody else.
2 Pretty much any HA network allows local association so N number of switches/dimmer can act as one. Z-Wave Zigbee etc all support this basic function. Some random wifi fly by night special not so much. Plenty of kit can support existing N way setups if you just want on/dimm last level and off Zooz makes a lot of these.
3 Not realy and please check to see that it's UL or whatever your local equivalent is.