Is an expiation of feeding 800 needy persons obligatory due to repeatedly swearing not to return to fornication and then returning to it, or is this considered a single oath because it was repeated with the same girl, and what must be done if one is unable to feed them?

What you are in, namely falling into indecency, is greater than expiation for an oath. What is obligatory upon you is sincere repentance from it, by abandoning it, regretting it, resolving not to return to it, and cutting off all paths that lead to it. As for the oath you swore, it is a binding oath and not a vain oath. If you swear an oath and then break it, expiation becomes obligatory upon you. If you return and swear another oath and then break it, another expiation becomes obligatory, and so on. The expiation consists of three options, given as a choice: feeding ten poor persons, or clothing them, or freeing a slave. If one is unable to do these, then fasting three days for each oath becomes obligatory, and consecutive fasting is not a condition for this fast.