hi P------

Grounding. I live on the 3rd. floor and would need to run a wire 20-25 metres long to a tree in the back garden. Will that work?
Yes you could run the wire its a better option,,or some people run the wire to their electrical plug using the earth pin
its not as good but it will give you an earth..
Does the wire have to be made of steel, and if so, should it be stainless (containing chrome also)? Can it not be a normal copper insulated electrical wire?
Your local hardware store has the wire you need its fine Galvanized steel,,the type the use for tying up steel in concrete,,not much thicker than a midele piano string

My floor has a wool carpet with some sort of (synthetic) rubber backing. Would feet still need to be rested on wood?
No
Position of power unit above the antennae: This would be difficult, but it could sit on the top shelf of a nearby wardrobe with door open during use?
The unit needs to be on a shelf behind the ring,,extending the leads will weaken the output considerably,,is it possible to just lower the wardrobe shelf

The full moon is occurring here in two days' time. How long before and after the full moon do I have to abstain from using the device
Just the night of the full moon