Hi Stepan,
and welcome to the fun of integrating with Openhab. You seem to have big dreams and you are lucky as just about anything you ask for can be done in Openhab. I’d imagine most of us run some variation of the set up you describe.
First things first:
You should read this: How to ask a good question / Help Us Help You
You are asking a lot of questions and most are important BUT not for this thread!
Nonetheless, I will try to point you in the right direction but if you want to follow up please do so only on the topics related to the WAVIN9000 automation here. For the rest - read a bit more, then if you still don’t have an answer make a new topic/post - but you will find an answer in the forums 
Answer: yes
Answer: Yes the main controller only shows single LED (red/green) depending on status. And yes once you get this binding working with Openhab you can do just about anything. See current temperatures (measured by the thermostat units in the rooms), current set temperature (what temperature your thermostat is aiming at), hysteresis, wireless control signal levels, and many more. You have basically low level programming access to the guts of the system.
Answer: yes. This is where the power of openhab comes. You connect to your equipment and then automate whatever you want. You will likely have to do some scripting but in OH3 you can do that simpler with Python (never tried it myself…)
Answer: yes but you need to be careful a bit because you start sounding like you want to treat the floor heating like a radiator. Any floor heating system will have hours of response time to any change. The command to raise the temperature for example 5 degrees will be accepted immediately but t will be 3-4 hours before you feel it. I am not sure some of the fine controls you want to do are achievable with floor heating at all. Lowering the temperature when not home only makes sense if you will be gone for more than few days i think. Not for 8 hours while at work.
Answer: probably but you need to be specific as to what that turn control is.
Answer: This is topic for another thread. In general yes homekit integration is possible and if the docs say it work i believe it does. I myself don’t use it. See: HomeKit Add-on - System Integrations | openHAB
Answer: seem so to me. Follow up on this with other threads if needed after carefully reviewing this: https://www.openhab.org/docs/installation/openhabian.html
Answer: Correct
Answer: Correct
Answer: See some earlier posts here with example HW. For example:
What you have found seems like the right thing. Check the pins we described in the posts and if all matches and you are happy with the length of cable - by all means get the ready made solution.
Opinion: Danfoss and Nilan systems are also widespread in DK. Wavin is a sub-company of some Mexican multinational. They are big and widespread - don’t think it’s in DK only.
Opinion: If you get Wavin working why would you want to change? It seems fairly simple to me to add channels to the Wavin system and if you get one working in Openhab then you got them all.
Nice plan. See this for inspiration: https://community.openhab.org/c/tutorials-examples/10
And post there your solution when done
Cheers