OK, update on backsweetening experiments: assuming fermentation is more or less finished, I tried a few things on some "samples" I had left over from the racking . Since Stones is quite sweet - as I said above, probably as much as 20% sugar, I tried several combinations on 100ml samples:
10g white sugar/100 ml
10g honey/100 ml
20g honey/100 ml
My immediate impression was that just sweetening the wine to any degree greatly improved the taste - it's just not meant to be a dry wine, I guess. With sweetening, it is a LOT more palatable.
10g of sweetener was perhaps not quite enough for my taste, while 20g was just a bit too much. Sugar gave a neutral sweet taste, while the taste of the honey came through a bit too strongly for my liking.
So I reckon I will compromise and backsweeten the whole batch with about 15g sugar or honey per 100ml, assuming that minimal additional fermentation will occur now owing to the high ABV - just not sure whether to risk honey and hope that the honey aroma will calm down a little over time as I don't really want to taste the honey that strongly. Or maybe I will even mix sugar and honey for an additional compromise. Thoughts?