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coz if you add more sugar it will ferment out to be a dry cider (up to a limit!)
if you kill off the yeast, how do you make it fizz?
is there a recommended non-fermentable sugar or sweetener?

I tried an artificial sweetener once with elderflower pop and it worked, but had a slight but nasty aftertaste

'Real Cidermaking on a Small Scale' has some good info on how to get different finishes if you can find a copy. It's a few pages long with a few diagrams otherwise I'd copy it out here, but feel free to PM me an email address and I'll happily scan the relevant pages for you.

Essentially it just involves very careful sweetening of the cider, the use of thick-walled bottles (champagne type ones), and leaving them to condition naturally (and not leaving them too long, I suppose cos you could end up getting bottle bombs). It also mentions using sweetener to get the sweet finish and a small amount of sugar to get the fizz (like priming beer).

Make a strong, fizzy, dry cider and dilute with apple juice in the glass for sweetness. Or sweeten in the glass with a drop of sugar syrup.

Or, there are several artificial/nonfermentable sweeteners you could try. If you don't like one, try another.

you could stop the fermentation early, so it's still sweet, and then force carbonate, either in pressure vessel with CO2 gas (sparklet) or by the bottle in a soda stream type way.

otherwise, once fermentation is finished, bottle into either pop bottles that have had pressure before or screw top wine bottles, or crown capped beer bottles (or thick walled champagne bottles if you want), add a teaspoon of fermentable sugar to each pint of liquid - add artificial sweetener to taste(1-8 tsp per gallon). Leave somewhere warm for two weeks, for the yeast to ferment the sugar to produce the CO2, then leave somewhere cold for two weeks - to help the CO2 absorb into your cider - serve chilled.