1) Haven't crossed this bridge yet! I see award mileage is deducted pro rata between the different partners, but doubt that this would apply in this scenario. I suppose they divvy up the mileage in the ratio it was earned, but this is merely an assumption.

2) I am very close to somebody who is a Swiss member and gets Swiss benefits, currently including effectively full mileage in Q class between ZRH and LHR, whereas I, a mere mortal, am resigned to earning just 125 miles.

She uses her Swiss address, of course, which she has used since she joined. For a change of address, other posts have suggested contacting the local EC office, in your case Sudbury.

I don't think they are particularly bothered about where travel begins and ends. Much of mine begins and ends in Vienna - partly because I have business there and partly because it suits me.

They do monitor travel. I had a bonus offer after they had noticed I had made several journeys starting from LHR but apparently not returning (at least, not with BA)! I was happy to accept the offer and fly BA both ways the next time for bonus mileage.