1) Yes, you can pay it using the existing money (i.e. your intention is still to pay the previous year’s khums). You would still need to determine if you have khums for the current year and the previous year’s payment will not be deducted from the savings calculation.
2) It is always best to check to see if a given organization has a written ijaza, for this reason, most post it on their website. If it turns out you only acted based on what you heard and they don’t have ijaza then it will be as if you did not pay it.
3) Yes, if they remain unused for one year then you must pay khums on them.