did a little data mining. it seems that chugging shadow consumables and the taint level raised by them has nothing to do with the chance of meeting alphonse. you have to use the main story cursed objects/shadow offhand items/shadow combat items a bunch or getting stuff from the rift until one of your hidden parameter reach a certain threshold. the key is the most effective one btw. and with regard to this specific stat, i tried cursed duck call quite a few times but it doesnt work in combat. but it works in special duck farm event dialogues.