For grouping things, I usually add the “Pulse” Add-on, and create a one-second pulse for each group of things I want to turn on or off (or to change colors in groups or whatever). For example, create a rule with a pulse input of “Event ‘turned on’ (or off)” is used to trigger a group of lights and/or outlets. Then create other rules to associate physical switches to those same one-second pulses that turn groups of things on/off. I often use a single press for “on” and a double press for “off”. In fact, I always create a one-second pulse called “everything” where I “invert” the pulse so that it’s normally on. When it is triggered to go off, then the rule turns associated things off. That way my voice command of “turn everything off” makes sense and in fact causes everything to turn off.
Of course you can turn everything on/off with the WebThings Gateway UI loaded in any browser, and the WebThings Gateway is 100% open source, nothing proprietary. The intent is privacy, security, and interoperability for all – putting people first.
Yes, you can remap the IKEA 5-button remote to things in the WebThing GUI. Just use the Rules engine to map the buttons to things you want (including to “pulses” that map to a group of things.)
It all works whether or not the RPi has a connection to the Internet. You just can’t command and monitor things from a remote location if the RPi is local only.
I find that the Dresden Elektronik Conbee II Zigbee dongle works really well with IKEA, Samsung SmartThings, and other Zigbee devices. Wi-Fi devices also work but it’s harder to make sure they are blocked from also connecting to the Internet directly if you don’t want them to (unless you block via rules on your firewall).
Good luck, enjoy.