In-floor radiant heating hooked up to hot water tank in new home - questions about how it works and operating it
Here is the way a system like yours should work. The heating tank, tank #2, is set to maintain a set water temp. That temp is determined by how the water temp is maintained. It could have a programming sensor that adjusts the temp up or down depending on the outdoor temp or could have a manual set point usually around 125 degree F max. Now to answer your questions:  Q #1,the pipe at arrow #2 should be the same temp as the pipe at arrow #3 unless it's pump is running. Q #2 I have to surmise that the "flow control valve" that prevents gravity flow is not working as it should be. Those 2 red colored devices after the green expansion tank have levers located on the top. They both need to be closed. Turn the lever on each CW to close or look on the device to see if any instructions are printed on it to show the direction of closed. This will or should stop the gravity flow you have from question #1.  Question #3 depends on the : code " in your area. Yes, it is a closed loop and should not need any make-up water. The part I do not like is that there is no "back flow preventer" installed on the make-up line to prevent the water in the heating system from contaminating the domestic water in the other tank. Sometimes, we would have a special fitting that we could isolate both tanks by removing the connecting line between them and reconnect in the event we had to add water to the heating system. For Q #4 "A" is yes or no, I prefer NO; B is yes; C is yes or no, refer to Q #3; D is NO; Q #5 is never;  Q #6 should be NO, since each thermostat should control it's own zone. Hope this helps