Is it permissible to offer precautionary expiations for oaths with the intention of charity for those that have been forgotten, and will these precautionary expiations suffice for those that are remembered later, and is it sufficient to estimate the number of expiations and offer them without remembering the specific oath for each one?

You must pay what you believe will most likely cover the necessary expiations, and any excess will be considered charity according to your intention. If the subsequent expiation falls within what you initially set aside, then no further expiation is required. However, if it falls outside of what you intended as charity, then you must pay another expiation, because what was originally given was with the intention of a voluntary act, not an obligatory one. There is no impediment to estimating the number of obligatory expiations; rather, it is necessary to pay expiation for the number of oaths you broke, based on your best judgment, without needing to know their specifics. A Muslim must guard his tongue from excessive swearing by Allah Almighty.