You can buy sugar free elderflower cordial from Sainsbury's (I found to my delight) sadly it doesnt taste the same as Chardonnay for which I was trying to replace by buying it.
I suppose you can, but I think it wouldn't keep very well, as the sugar is a preservative. If you have enough room in your freezer, though, and use what's thawed in a couple of days, I don't see why not
My mom makes something she jokingly calls dry buz (buz being the name of elderflower back home). Although the recipe contains sugar, I don't see why it can't be made without.
Collect and dry (naturally) elderflower blooms. In 5 l. Water soak several dried flowers for 24 hours (or more). Grate over lemon peel. Mix with sugar or in this case sweetener and you are done.
Hope it helps