Is it permissible to pay the fidyah for Umrah – for abandoning the state of ihram from the miqat for one who intends to go to Madinah first and then enter ihram from Abyar Ali – to a charitable society in Makkah that slaughters on behalf of the donor, and is it valid to fast or feed in lieu of slaughtering?

No blood sacrifice (dam) is incurred by revoking the intention of ihram before reaching the miqat, so your ihram from Dhu al-Hulayfah is valid and preferable. The hady (sacrificial animal) is obligatory for whoever is required to slaughter it and distribute it to the poor of the Haram, and if that is not possible, then it should be given to a trusted charity. Nothing is incumbent upon you due to your aforementioned action.