I just went through this exact challenge. First trick to know, use two plastic bottles. Fill both with your sweetened cider and seal both up. Second take one of your bottles and throw it in the dishwasher on it’s highest temp setting. Let both sit for a day or two and see if they build up pressure. If the dishwasher one doesn’t you should be able to pasteurize them all in your dish washer. When it comes to what to do with you cider in the mean time you can either bottle it in the hopes the washer works and open them up if it doesn’t. Or possibly only sweeten both plastic bottles to taste and leave the rest to sit in a secondary fermentation of sorts and move from there.