(1) Is sulphur dioxide haram to consume? What about L-cystine?
(2) If a husband does not pay khums, is the wife required to pay it even if she does not work and has no source of income?
1) If you do not know the source of the preservatives in question, you can consider them pure and they are permissible to consume.
2) Your husband is responsible for his khums (i.e., from his earnings). You can encourage and convince him to practice his religious obligation. As far as yourself, you do not have to pay khums unless you gain something that you do not need or do not use for a whole year.