GSH & HW. novice questions .pls help

I've been in my current house for 5 yrs but never bothered looking at my CH system, but recently my pump failed ( impeller snapped of shaft ). I think this is probably an effect of the system not running properly however, so I've decided to pay attention to it and figure out what should & shouldn't be happeneing. In order to try and get some clear answers, I've done some sketches and provided links to support my questions :- arrangement.pdf & Thermo diagrams.pdf layout.pdf
Q1. Does the hot water circuit use pump or just gravity?
Q2. Should switching to HW on my control unit make the motorized valve shut ( & trip off the pump if answer to item #1 is gravity )?
Q3. On the valve, does switching from auto to manual make valve shut or stay open?
Q4. Is one of the 22m pipes in the piping arrangement just the vent to my header tank?
Q5. Why have a -15-+10 thermostat in the garage? , but none in the house , and in which parameters does it work.
Q6. Further #5 I don't have an internal thermostat. Would I benefit from intalling one> Please describe where it should be hooked up to.
Q7. Instead of an internal thermostat, would a string of thermostatically controlled radiator valves make more sense?
Q8. Roughly how much would a thermostat cost compared to individual radiator valves?
Q9. What point do you typically drain the system from ? Should there be a 'y' valve somewhere that I could stick a hose pipe on?
Q10. I've never attempted to put any corrosion inhibitor into the header tank and there's no filtration of any kind inthe system. Should there be either ?
Q11. For future maintenance of pump/valve/boiler, would my system benefit from some isolation valves installed on the garage wall per my piping sketch. Please advise where I might put one or two valves to avoid having to frequently drain the whole system when things go wrong.
More questions will result from the answers I receive
Please cc newsgroup responses to
Thanks in advance

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