Yes. Our boiler is slightly older and doesn't have the fine control that newer ones have, but that is also standard and if your boiler has it, the controllers will use it too. On the Wiser system, each TRV signals to the controller what % heat it wants and the controller consolidates all of the requests and, if above an internal threshold, it demands heat from the boiler. Our boiler is a gas combi but I think other types may also be supported.
No, and if you did that, you would need to be incredibly careful. Also, your insurers and the local council would probably not be happy. Really, these things need to be certified. The systems already have a lot of "smarts" built in including weather sensing. The Wiser system is actually quite basic, the Evohome is a lot smarter I believe.
You can have many stand-alone thermostats. We have only 1, in the bathroom where there is no TRV on the radiator and where we also have under-floor heating (electric so not part of the Wiser system). The thermostat also measures humidity.
Most rooms just have the smart TRV on the radiator. This senses and reports temperature of course.
The thermostat/TRV actually does that locally and, as mentioned above, reports its "demand" requirement back to the controller.
There are 2 modes on the Wiser controller. The simple mode (the only one when we started) simply measures the combined demand and controls the boiler accordingly. Works pretty well but you have to manually allow for room/building warm-up time. The other mode was introduced about a year ago I think. It uses weather information and learns the characteristics of your house so you set when you want a room to be at the chosen temperature instead of when you want to start the heating (the normal schedule) and it works out when to start the schedule.
Each room has its own schedule. Of course, you can override the schedule either by twisting the TRV to give a 30min boost/cool or via the app. Or via Node-RED and the REST API of course.
There is also an away mode that turns everything off but has a minimum temperature so that you can prevent the house from freezing if you are away in the winter. It is great to be on your way back from holiday when it is colder and to just turn on the heating from the car.
You will loose some of the ML smarts that way but certainly the Wiser and I believe the Evohome will continue to work perfectly without the remote server. If you are in the house, the app first tries to connect via the cloud and if that fails, it tries to connect locally so you can still use the app even if you block cloud access. But of course, using Node-RED, you connect direct to the controller.
You can't really do without the controller box though as it ties everything together and provides the REST API and the Zigbee interface.
As mentioned, personally, I would not want to build my own home heating controller. There is too much at stake (e.g. lives as well as the house and potentially your neighbours houses and lives). Really not worth the risk for the low cost of the controller. You would almost certainly invalidate your house contents and your building insurance as well. Indeed, in most parts of the world, it is probably illegal? Just like you have minimum certified building codes for electrics.