Let me first reply to this thread and I’ll cover any remaining questions from the other thread afterwards.
That is correct. To decouple the UI from the backend we use MQTT and websockets. You can launch the UI subscribing to a different MQTT server instead of localhost and even different topics.
apt updates should be fine but we haven’t tried it yet.
Coming from an update to 2.4 with all that was happening on MQTT we would never suggest it on a live instance. Obviously you know exactly what 2.5 affects in terms of functionality and syntax so go ahead but we strongly discourage beginners to be the first to try it.
Haven’t used it before. How can you react to specific file changes? Is it system-specific or OH-specific catches you are after?
I feel a total newb to openHAB compared to your experience and exposure! I need to do some homework tonight on these subjects but having coded scenarios with the existing rules engine I totally felt the lack of a higher level language to code. Rules have been the hardest bit to process during boot for the Pi Zero W but it was mostly CPU intensive rather than memory. We actually have a little trick to load a very basic rules set initially and when system is fully up, then load the actual full set of rules. Still boot time is toooo long. If you feel this will improve this you are the most appropriate person to take this decision and dig in.
Didn’t know of restoreOnStartup till now. We have already a few issues open and one is addressing the lack of non volatile settings. If this is something you want to address too please feel free, but I feel like I gave you my month’s worth of tasks from day 1.
At the moment only the second stage heating has these configurable (Settings>2nd Stage Heating>) but another user has mentioned it and it makes total sense. We feel some functionality slipped while we were debugging, manufacturing and chasing slipped delivery dates. 
. Rule checkcurrtemp is the place for anyone else who want to hardcode it, but you know that already 
We thought of having a switch between an ECO (higher) and a Comfort (lower) hysteresis mode under settings. Having their actual values configurable from the UI or not is simply a matter of balance between simple UI and customisation.
More homework here 
We saw this error but we couldn’t something about it. Suppressing it didn’t appear as an option then.
From your previous thread I kept in the ToDo list the obvious need for documentation on the UI which I will work on this weekend.
It is calling the openhab-cli to backup all openHAB files. Not system.
Please respond here with your view on all the above. It is a lot!
I cannot thank you enough for your valuable insight on our project.