Or the system just sees that you are coming home (iCloud binding, or OwnTracks, or IFTTT, or BT sensors, etc), knows it is dark (Astro binding, light sensors, etc), and does all of this automatically without being prompted…
Absolutely possible given the information you have provided so far. Like Vincent said, you will want to look at the HTTP binding first.
Yes.
It is often better to get some sort of feedback from the device because then you can manually change the state of the device and your home automation’s state will remain consistent with reality. But you can make it work without. You just won’t be able to fully rely on your home automation state matching the real state. That may or may not matter depending on the context. Note that the feedback need not come from the device itself. For example, I have a relay to control the opening/closing of my garage door and a reed sensor to tell me whether the door is open or closed.
I don’t know much about Homkit integration, but I suspect when you ask for the status of something, the state of the Item that represents that thing in your home automation is what gets returned. In my garage door example, the state of the Item that represents the reed sensor is what gets sent to Homekit in response.
4 relays, 4 inputs, and up to four sensors? These are awesome devices! I’m guessing that the relay1state is representing the state and a 1 indicates ON.
Certainly. It is valid XML so you would probably use the XPATH or XSLT transform. If you ommit the “?relay1State=2” do you get this XML back without changing anything on the device? That would be good because then you can poll the device and capture manual changes to the device’s state.
This is just something you will have to accept. 
Even the best of us will head down a path and reach a point where we learn something new or encounter an untenable problem that will force a backtrack and rebuild. But that is how we learn and that is how we make our systems better. I’ve probably rebuilt my rules at least three times now.
On-the-other-hand, there is no wrong way to do any of this stuff. If you go down a path and it works, who cares if there is a better way to do it. What you have works.
Those who have been most successful with OH are those who just dive right in and try things out. Go play and come back when you can’t figure something out.
So there is an X-310 2.x version binding? Awesome! You don’t even need to mess with the HTTP binding. The call to that URL is already handled for you. You just need to link Items to the Channels on those Things.
I recommend defining your Items in .items files. The UIs do not yet support tags and IIRC Homekit integration requires tags.
Not mad at all. It is actually pretty standard. I think you will have a pretty easy go of it compared to some.
I suggest installing the Demo package and looking at how all the pieces of OH work together. It is easier to take something that works and make minor changes and seeing how the change affects things.
Definitely go through the docs and you must read them closely. Very minor details matter, expecially things like case, spaces, etc).
Start small and work your way up.
Ask questions on the forum. When you do ask, please post all the relevant code (Items, Rules, Logs, etc) and How to use code fences.
Good luck!